QC 
896 

M3 
1912 


472. 

.  S.  DEPARTMENT  OF  AGRICULTURE, 

WEATHER     BUREAU. 


UIIOMETERS  AND  THE  MEASUREMENT 
OF  ATMOSPHERIC  PRESSURE. 


A  PAMPHLET  OF  INFORMATION  RESPECTING  THE  THEORY  AND 

CONSTRUCTION  OF  BAROMETERS  IN  GENERAL,  WITH 

SUMMARY  OF  INSTRUCTIONS  FOR  THE  CARE 

AND  USE  OF  THE  STANDARD  WEATHER 

BUREAU  INSTRUMENTS. 


IIKITLAK  F.  IXSTRVMKNT  DIVISION. 

KlUTHiN, 


BY 
C.  F.  MAKVIN, 

PROFESSOR   OP  METEOROLOGY. 
Prepared  under  direction  of  WILLIS  L.  MOORE,  Chief  U.  S.  Weather  Bureau. 


WASHINGTON: 

GOVERNMENT  PRINTING  OFFIOKL 
1912. 

I 


W.  B.  No.  472. 

U.  S.  DEPARTMENT  OF  AGRICULTURE, 

WEATHER     BUREAU. 


BAROMETERS  AND  THE  MEASUREMENT 
OF  ATMOSPHERIC  PRESSURE. 


A  PAMPHLET  OF  INFORMATION  RESPECTING  THE  THEORY  AND 

CONSTRUCTION  OF  BAROMETERS  IN  GENERAL,  WITH 

SUMMARY  OF  INSTRUCTIONS  FOR  THE  CARE 

AND  USE  OF  THE  STANDARD  WEATHER 

BUREAU  INSTRUMENTS. 


rillCULAR  F,  INSTRUMENT  DIVISION. 

FOURTH  EDITION, 


BY 


C.  F.  MARVIN, 

»  i 

PROFESSOR    OF    METEOROLOGY. 


Prepared  under  direction  of  WILLIS  L.  MOORE,  Chief  U.  S.  Weather  Bureau. 


WASHINGTON: 

GOVERNMENT  PRINTING  OFFICK. 
1912. 


8CIENCW 


PUBLICATION  may  be  pro- 
-*-     cured  from  the  Superintendent  of 
Documents,  Government  Printing  Office 
Washington,  D.  C.,  at  15  cents  per  copy 


EARTH 

SCIENCES 
LIBRARY 


LIST  OF  ILLUSTRATIONS. 


I'age. 

1,2,3.  Mecurial  barometer,  with  Fortin  cistern 11 

4,  5,  6,  7,  8.  Verniers 13 

9.  Tuch  barometer  cistern 16 

10.  Fixed  cistern  barometer 17 

11.  Tube  and  cistern  of  marine  barometer 20 

12.  Marine  barometer  and  box 20 

13 .  Funnel  method  for  filling  barometers 22 

14.  Boiling  method  of  filling  barometers 22 

15.  Air-pump  method  of  filling  barometers 24 

16.  Aneroid  barometer 32 

17.  Goldschmidt's  aneroid 33 

18.  Dial  barometer 37 

19.  Sympiesometer 38 

20.  Howson's  barometer 38 

21.  Three-liquid  barometer 38 

22.  Foreman's  barograph 41 

23.  Marvin's  normal  barograph 44 

24.  Electric  motor  mechanisms 45 

25 .  Compensated  siphon  barograph,  Marvin  system 47 

26.  Three-piece  siphon  tube 47 

27.  Time  marker 55 

28.  Richard's  barograph 56 

29.  Improved  barometer  box 59 

30.  Barometer,  cistern,  and  ring  support 60 

31.  Old  style  barometer  box 61 

3 


171 


LETTER  OE  TRANSMITTAL 


U.  S.  DEPARTMENT  OF  AGRICULTURE, 

WEATHER  BUREAU, 
Washington,  D.  C.,  January  3,  1912. 

SIR:  I  have  the  honor  to  transmit  herewith  the  revised  copy  of  a 
pamphlet  prepared  by  Prof.  C.  F.  Marvin,  containing  information 
and  instructions  to  the  observers  of  the  Weather  Bureau  for  the  care 
and  use  of  barometric  apparatus,  and  to  request  that  it  be  published 
as  Circular  F,  Instrument  Division,  fourth  edition. 
Very  respectfully, 

WILLIS  L.  MOORE, 
Chief  U.  S.  Weather  Bureau. 
Hon.  JAMES  WILSON, 

Secretary  of  Agriculture. 

5 


NOTE  TO  OBSERVERS. 


U.  S.  DEPARTMENT  OF  AGRICULTURE, 

WEATHER  BUREAU, 
Washington,  D.  C.,  January  8,  1912. 

The  details  of  information  respecting  barometers  contained  in  the 
accompanying  pages  are  furnished  for  the  instruction  and  guidance 
of  observers,  who  will  keep  themselves  informed  upon  the  subjects 
treated,  and,  in  particular,  will  observe  the  technical  duties  relating 
to  the  care  and  use  of  barometric  apparatus  summarized  for  their 
convenience  in  Section  VII.  The  provisions  of  this  section,  as  more 
fully  set  forth  in  the  body  of  the  pamphlet,  will  replace  instructions 
on  barometers  heretofore  issued. 

WILLIS  L.  MOORE, 
Chief  U.  S.  Weather  Bureau. 


COXTKXTS. 


Page. 

Introductory — The  barometer 9 

I. — Mercurial  barometer 10 

II. — Aneroid  barometers 31 

III . — Miscellaneous  barometers 36 

IV.— Barographs 39 

V. — General  instructions 57 

VI. — Concerning  the  elevation  of  stations 81 

VII. — Summary  of  special  instructions  for  observers  of  the  Weather  Bureau. .  85 

VIII.— Numerical  tables 88 

Index 105 

7 


BAROMETERS  AND   THE   MEASUREMENT   OF  ATMOSPHERIC 

PRESSURE. 


INTEODUCTOEY. 
HIE    BAROMETER. 

1.  The  hydrostatic  principle,  by  virtue  of  which  the  pressure  of  the 
air  is  measured  by  the  ordinary  barometer,  was  first  formulated  at 
Florence  in  1643  by  Torricelli,  whose  famous  experiments  demon- 
strated, not  only  that  the  air  exerted  a  very  great  pressure,  but  that 
this  pressure  changed  slightly  from  day  to  day. 

12.  Torricelli' s  barometer. — To  repeat  Torricelli's  experiment,  fill 
a  clean,  dry,  preferably  warm,  glass  tube,  closed  at  one  end,  with  pure, 
dry  mercury,  using  care  to  exclude  all  air.  The  length  of  the  tube 
must,  in  general,  exceed  30  inches.  Close  the  open  end  of  the  tube 
firmly  with  the  finger  tip,  and  submerge  it  in  an  open  cup  of  mer- 
cury. Upon  removing  the  finger  and  causing  the  tube  to  stand 
vertically,  a  portion  of  the  mercury  will  pass  from  the  tube  into  the 
cup,  leaving  a  vacuous  space,  known  as  Torricelli's  vacuum,  in  the 
top  of  the  tube.  The  column  of  mercury  remaining  in  the  tube  will, 
at  sea-level  stations,  be  about  30  inches  high.  The  weight  of  this 
mercury  is  sustained  by  and  exactly  balances  the  downward  pressure 
of  the  air  upon  the  surface  of  the  mercury  in  the  cup.  The  height  of 
such  a  mercurial  column,  therefore,  becomes  a  measure  of  the  pressure 
of  the  air,  and  Torricelli  seems  to  have  been  the  first  to  discover  that 
the  height  of  such  a  column  varied  from  day  to  day. 

3.  Siphon  barometer. — Instead  of  constructing  the  barometer  in  the 
manner  just  described,  where  the  cistern  and  tube  are  in  separate 
parts,  the  tube  may  be  made  longer  and  turned  up  at  the  bottom  so 
as  to  resemble  the  letter  J,  forming  what  is  commonly  called  a  siphon 
barometer,  the  long  arm  of  which  is  closed  at  the  top. 

4.  Pressure  of  one  atmosphere. — Suppose  the  area  of  the  inside  of  the 
barometer  tube  to  be  just  1  square  inch,  then  a  30-inch  barometric 
column  will  contain  just  30  cubic  inches  of  mercury.     Now,  1  cubic 
inch  of  mercury  weighs  0.4906  pound,  which,  multiplied  by  30,  gives 
the  ordinary  sea-level  pressure  of  the  air  to  be  14.718  pounds  per 
square  inch.     This  quantity  is  frequently  used  by  engineers,  and  is 
called  a  pressure  of  one  atmosphere. 

9 


10        BAROMETERS   AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

5.  This  pressure  of  14.7  pounds  per  square  inch  is,  in  the  main, 
nothing  more  than  the  weight  of  an  air  column  having  a  sectional 
area  of  1  square  inch  and  extending  vertically  to  the  upper  limits  of 
the  atmosphere.     In  addition  to  the  weight,  pure  and  simple,  how- 
ever, such  influences  as  the  wind,  the  rapid  heating  and  cooling 
in  confined  layers  of  air,  and  other  causes  modify  by  small  amounts 
the  elastic  pressure  of  the  air. 

6.  Other  forms  of  barometers. — Within  the  past  50  or  60  years  a 
form  of  barometer,  made  entirely  of  metal,  has  been  devised,  and  is 
widely  used  at  sea  and  by  tourists  and  others  on  account  of  its  con- 
venience and  portability.     This  form  is  commonly  designated  the 
aneroid,  a  word  which  signifies  " containing  no  liquid."     The  instru- 
ment is  also  often  called  the  holosteric  barometer,  meaning  ' '  wholly 
of  solids."     Aneroids,  though  often  highly  sensitive,  are,  at  best, 
much  less  accurate  than  properly  constructed  mercurial  barometers, 
as  will  be  explained  later. 

.   .  > 

I. — MERCURIAL    BAROMETERS. 

., 

7.  Fortin  barometers,   Weather  Bureau  pattern. — In  order  that  the 
height  of  the  mercurial  column  may  represent  accurately  the  true 
pressure  of  the  air,  and  in  order  to  detect  the  comparatively  small 
changes  of  pressure  from  day  to  day,  many  refinements  are  necessary 
in  the  construction  of  the  instrument  and  great  precision  of  measure- 
ment is  required.     An  excellent  form  of  the  mercurial  berometer, 
satisfying  the  requirements  just  stated,  was  devised  by  Fortin,  and  is 
now  very  widely  used  the  world  over.     The  particular  pattern  used 
by  the  Weather  Bureau,  sometimes  called  the  Green  barometer,  is 
figured  on  page  11. 

8.  The  barometer  consists  of  a  glass  tube,   about  J-inch  inside 
diameter,  closed  at  the  top  and  inclosed  in  a  thin  metal  tube,  through 
which  large  openings  are  cut  on  opposite  sides,  exposing  to  view  the 
glass  tube  and  mercurial  column.     The  graduated  scale  is  formed 
at  one  side  of  this  opening,  and  a  short  tube  or  sleeve,  also  gradu- 
ated (shown  at  C,  figs.  1  and  2),  encircles  the  barometer  tube  and 
slides  smoothly  within  the  metal  part,  motion  being  given  to  it  by 
means  of  the  milled  head,  D,  and  a  small  rack  and  pinion  inside. 

At  E,  figure  2,  is  shown  what  is  called  the  attached  thermometer. 
The  bulb  of  this  is  entirely  concealed  within  the  metal  tube,  and  is 
between  it  and  the  glass  barometer  tube,  so  as  to  show  as  nearly  as 
possible  the  mean  temperature  of  both  the  brass  tube  and  the  mer- 
cury. 

9.  Cistern. — The  special  feature  of  the  barometer  is  a  cistern  so 
constructed  that  the  level  of  the  mercury  within  may  be  changed 
greatly  and  adjusted  to  a  fixed  index  point. 


BAROMETERS    AND   MEASUREMENT   OF   ATMOSPHERIC    PRESSURE.        11 

The  topmost  portion  of  the  cistern  consists  of  a  small  boxwood 
piece,  Gj  figure  3.  The  glass  tube  t,  passes  through  the  central  portion 
of  this,  to  which  it  is  secured  by  a  piece  of  soft  kid  leather  folded  in 
a  peculiar  manner  and  securely  wrapped  to  both  the  glass  tube  and 
the  boxwood  cap,  G.  The  flexible  joint  thus  formed  will  not  allow 
the  mercury  to  escape,  but  permits  the  passage  of  air  to  and  from 
the  cistern. 

The  remaining  portions  of  the  cistern  are  the  short  glass  cylinder, 
F,  figure  3,  the  two  curved  boxwood  pieces,  i  and  j,  and  the  kid 
leather  bag,  N,  with  adjusting  screw,  0,  clamps,  etc. 

It  is  plainly  seen  that  on  turning  the  screw,  0,  the  leather  bag  may 
be  folded  up  into  or  withdrawn  from  the  curved  boxwood  chamber,  y, 
in  a  manner  to  cause  any  desired  change  in  the  level  of  the  mercurial 
surface. 

10.  Ivory  point. — At  h,  figure  3,  is  shown  what  is  technically  called 
the  "ivory  point/'  which  projects  downward  from  the  top  of  the  cis- 
tern and  forms  a  fixed  and  definite  point,  to  which  the  level  of  the  mer- 
cury in  the  cistern  can  be  adjusted  in  taking  readings  of  the  barometer, 
as  will  be  described  hereafter. 

The  ivory  point  is,  therefore,  the  zero  end  of  the  scale,  from  which 
all  the  measurements  of  the  height  of  the  column  are  made. 

1 1 .  Scale  of  barometer. — The  scale  of  the  barometer  is  seen  on  the 
left  of  the  opening,  at  the  top.     It  is  most  conveniently  made  of  a 
separate  strip  of  metal,  although  sometimes  it  is  engraved  directly 
on  the  metal  tube  itself.     The  length  varies  from  about  4  inches,  for 
use  at  stations  of  only  moderate  elevation  above  sea  level,  to  from  10 
to  15  inches,  or  more,  for  barometers  intended  to  be  used  in  balloons 
or  on  lofty  mountain  summits.     The  graduations  on  the  scale  also 
vary,  being  only  10  spaces  to  the  inch  in  many  instances  and  20  in 
others;  the  latter  graduation  is  to  be  preferred  on  account  of  the 
greater  accuracy  attainable  in  readings. 

The  scale  of  the  barometer  when  engraved  on  a  separate  strip  is 
attached  to  the  metal  tube  by  small  screws  in  such  a  manner  that  it 
may  be  adjusted  slightly  up  and  down,  so  that  the  30-inch  mark,  for 
example,  of  the  graduations  can  be  placed  at  exactly  the  right  dis- 
tance from  the  ivory  point.  This  adjustment  being  made,  the  scale 
should  not  be  moved  afterwards. 

12.  Vernier. — A  vernier  is  a  device  by  which  one  is  able  to  ascer- 
tain accurately  much  smaller  fractional  subdivisions  of  a  graduated 
scale  than  could  otherwise  be  observed  by  the  eye  without  the  aid 
of  a  microscope.     For  example,  with  a  scale  having  only  20  subdi- 
visions to  the  inch  a  vernier  enables  us  to  ascertain  accurately  the 
one-thousandth  part  of  an  inch.     The  name  of  the  device  is  derived 
from  its  inventor,  Pierre  Vernier.     This  portion  of  the  barometer  is 
the  little  graduated  scale,  C,  figures  1  and  2. 


12        BAROMETERS  AND  MEASUREMENT   OF   ATMOSPHERIC   PRESSURE. 


FIG.  l. 


o 

FIG.  2. 
Mercurial  barometer,  with  Fortin  cistern 


FlO.3. 


BAROMETERS   AND   MEASUREMENT   OF   ATMOSPHERIC    PRESSURE.        13 

A  vernier  consists,  essentially,  of  a  small  graduated  scale,  the 
spaces  upon  which  are  just  a  certain  amount  smaller  or  larger  than 
those  on  the  main  scale.  When  two  such  scales  are  placed  together 
some  particular  line  of  the  one  will  always  be  coincident,  or  very 
nearly  so,  with  a  line  on  the  other,  and  from  this  circumstance  the 
position  of  the  zero  line  of  the  vernier  in  reference  to  the  scale  can 
be  very  accurately  determined,  as  will  be  readily  understood  from  a 
study  of  the  following  figures  and  explanation: 


30.177 
FIG.  8. 


Figure  4  exhibits  the  manner  of  graduating  a  vernier  so  as  to  sub- 
divide the  spaces  upon  the  scale  into  tenths.  In  the  figure,  6  is  the 
scale  and  a  is  the  vernier.  The  lower  edge  of  the  vernier,  which  in 
this  case  is  also  the  zero  line,  is  exactly  opposite  or  coincident  with 
30  on  the  scale.  The  tenth  line  on  the  vernier  is  coincident  with  the 
ninth  line  above  30 — that  is,  a  space  of  9  divisions  on  the  scale  is 
divided  into  10  spaces  on  the  vernier,  so  that  each  space  on  the  latter 
is  one-tenth  part  shorter  than  a  space  on  the  scale.  In  the  present 
case  the  spaces  on  the  scale  represent  inches  and  tenths;  hence  the 
difference  between  the  length  of  a  space  on  the  vernier  and  one  on 
the  scale  is  TV  of  TV  =  T^(r  of  an  inch.  This  principle  of  matching 
two  scales  having  spaces  of  slightly  different  magnitude  is  always 
followed  hi  the  construction  of  verniers,  though,  of  course,  the  num- 
ber of  spaces  embraced  by  the  vernier  is  varied  to  suit  the  circum- 
stances and  the  degree  of  minuteness  desired.  Moreover,  in  some 
instances,  the  vernier  embraces  one  more  space  on  the  scale,  instead 
of  one  less,  than  the  number  of  its  own  subdivisions — that  is,  10 
spaces  on  the  vernier  may  be  made  to  correspond  to  11  spaces  on 
the  scale. 


14        BAROMETERS    AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

If,  as  we  have  seen,  the  spaces  on  the  vernier  are  one-tenth  smaller 
than  on  the  scale,  then,  in  the  adjustment  shown  in  figure  4,  the  first 
line  above  the  zero  on  the  vernier  is  one-tenth  part  of  the  space,  the 
next  line  two-tenths,  the  next  three-tenths,  etc.,  distant  from  the 
line  next  above  on  the  scale.  When,  therefore,  we  find  the  vernier  in 
such  a  position  as  shown  in  figure  5,  where  the  fifth  line  on  the  ver- 
nier is  coincident  with  a  scale  line,  it'  is  very  clear  that  the  zero  line 
of  the  vernier  must  be  just  five-tenths  above  the  scale  line  next  below. 
Now,  since  we  imagine  these  scales  to  represent  niches  and  tenths, 
then  figure  5  will  read,  30.15  inches. 

13.  Estimation  of  fractions  on  a  vernier. — In  many  cases  it  will 
happen  that  no  single  line  on  the  vernier  will  be  exactly  coincident 
with  a  scale  line,  but  that  one  line  will  be  a  little  above  while  the 
next  line  on  the  vernier  will  be  a  little  below  the  corresponding  scale 
lines. 

In  the  case  shown  in  figure  6  the  seventh  and  the  eighth  lines  on 
the  vernier  are  each  nearly  in  coincidence,  but  neither  one  is.  exactly 
so.  This  indicates  that  the  reading  is  somewhere  between  30.27  and 
30.28.  Moreover,  we  can  clearly  see  that  the  eighth  line  is  nearer 
coincidence  than  the  seventh.  We,  therefore,  estimate  that  the  true 
reading  is  about  30.277.  We  might,  probably,  with  as  great  accuracy 
have  selected  30.278. 

If  the  scale  and  vernier  are  accurately  graduated,  such  readings 
by  a  practiced  observer  will  rarely  be  in  error  by  more  than  0.002 
inch.  It  is  important  in  estimating  the  fractions  that  the  eye  be 
exactly  in  front  of  the  lines  being  studied. 

14.  In  figures  7  and  8  are  shown  verniers  applied  to  a  barometer 
scale  having  20  parts  to  the  inch.    In  this  case  24  parts  on  the  scale 
are  divided  into  25  parts  on  the  vernier.    By  the  principle  already 
explained  in  paragarph  12,  the  value  of  the  subdivisions  effected  by 
such  a  vernier,  or,  as  it  is  most  frequently  expressed,  the  least  count 
of  the  vernier,  will  be  fa  of  fa  =5^  of  an  inch.     In  reading  the 
vernier,  therefore,  each  line  will  represent  0.002  inch,  so  that  the 
fifth,  tenth,  fifteenth,  twentieth,  and  twenty-fifth  lines  will  repre- 
sent one,  two,  three,  four,  and  five  hundredths  of  an  inch,  respec- 
tively, and  are  so  numbered. 

As  described  in  paragraph  13,  the  lines  in  this  kind  of  vernier  also 
may  not  be  exactly  in  coincidence;  but  in  such  a  case,  owing  to  the 
smaUness  of  the  spaces,  it  is  not  of  any  special  advantage  in  making 
our  estimate  to  consider  whether  coincidence  is  nearer  one  line  than 
the  other.  In  ordinary  practice  we  simply  take  midway  between. 
Thus  m  figure  8  the  reading  is  between  30.176  and  30.178;  we  there- 
fore adopt  30.177  as  the  proper  reading. 

15.  Caution  against  error. — When  the  zero  line  of  this  style  of  ver- 
nier is  next  above  one  of  the  shortest  lines  on  the  scale,  as  was  the 


BAROMETERS   AND  MEASUREMENT   OF   ATMOSPHERIC   PRESSURE.        15 

case  in  the  example  above,  some  attention  is  necessary  in  order  to 
take  off  the  correct  reading.  For  example,  in  figure  8  we  find  that 
coincidence  on  the  vernier  is  between  lines  designated  26  and  28, 
which  corresponds  to  a  reading  of  0.026  or  0.028,  or,  taking  midway 
between,  0.027.  On  the  scale  itself,  however,  we  see  the  graduation 
next  below  the  first  line  of  the  vernier  is  30.150.  The  complete  read- 
ing is  found  by  adding  the  parts  thus:  30.150  +  0.027  =  30.177.  It 
frequently  happens  with  beginners  that  the  0.050  represented  by  the 
short  line  on  the  scale  is  overlooked  and  omitted  entirely — that  is, 
the  above  reading  might  be  called  30.127.  Whenever  readings  are 
made  with  a  scale  and  vernier  of  this  character,  special  pains  must 
be  taken  not  to  omit  adding  0.050  to  the  vernier  reading  when  the 
first  line  below  the  zero  of  the  vernier  is  a  short  one. 

16.  Strain  on  the  cistern  of  a  barometer. — When  the  mercury  is  sent 
up  to  the  top  of  the  tube  of  a  barometer  by  screwing  up  the  cistern, 
an  internal  hydrostatic  pressure  is  produced  proportional  to  the 
amount  by  which  the  length  of  the  column  has  been  increased. 
This  pressure  tends  to  force  the  mercury  through  the  joints  of  the 
cistern  or  the  joints  and  pores  of  the  leather  bag.     This  is  more  par- 
ticularly the  case  with  a  barometer  at  an  elevated  station,  where,  owing 
to  the  diminished  air  pressure,  the  column  may  need  to  be  raised  10 
or  15  inches  in  filling  the  tube,  greatly  endangering  the  cistern.     For 
this  reason  the  Fortin  barometer  cistern  is  not  wholly  satisfactory, 
as  it  is  difficult  to  make  and  keep  the  joints  so  tight  that  the  mercury 
will  not  be  able  to  find  its  way  through  some  very  small  crevices; 
such  leakage  soon  impairs  the  barometer. 

17.  Tuch  cistern. — Many  of  the  barometers  of  the  Weather  Bureau 
are  fitted  with  an  improved  form  of  cistern,  devised  by  Mr.  Charles 
B.  Tuch,  of  the  Instrument  Division.     The  construction  of  this  is 
shown  in  figure  9. 

The  chamber  for  the  mercury  is  formed  of  the  iron  cylinders,  c,  fc, 
provided  with  windows  at  the  top,  and  a  small  glass  cylinder,/.  The 
glass  barometer  tube  is  fastened  into  a  metal  piece,  6,  by  means  of 
several  thicknesses  of  leather  washers,  held  and  clamped  by  a  screw, 
e,  above.  The  piston,  o,  fits  the  cylinder  very  snugly  and  can  be 
moved  up  and  down  by  means  of  the  milled-head  screw,  W,  thereby 
adjusting  the  mercury  to  any  desired  level. 

18.  Fixed  cistern  barometers. — It  is  very  evident  that  as  the  column 
of  mercury  rises  and  falls  in  a  barometer  tube  there  is  a  corresponding 
change  in  the  level  of  the  surface  in  the  cistern,  and  as  long  as  the 
quantity  of  mercury  in  the  whole  barometer  remains  the  same,  it 
follows,  except  for  slight  temperature  effects,  that  the  true  height  of 
the  column  of  mercury  may  always  be  found  simply  from  readings  at 
the  top  end,  a  due  allowance  being  made  for  the  slight  rise  and  fall  in 
the  cistern. 


16        BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE. 


•y- 


m 


Adjustable  cistern  barometers,  such  as 
described  in  paragraphs  9  and  17,  are,  in 
general,  the  most  accurate,  as  the  cor- 
rection for  capillarity,  paragraph  26,  is 
usually  more  constant,  and  the  acci- 
dental escape  of  a  little  mercury  from  the 
cistern  does  not  matter.  Still,  very  ac- 
curate results  may  be  obtained  by  the 
use  of  well-made  barometers  with  fixed 
cisterns,  and  this  form  is  often  adopted 
in  the  construction  of  barometers  for  use 
on  shipboard. 

The  relation  between  the  true  length 
of  the  column  and  the  observed  position 
of  its  top  depends  upon  the  relation 
between  the  inside  areas  of  the  cistern 
and  barometer  tube.  When  this  relation 
has  been  once  worked  out  it  is  then  nec- 
essary in  reading  the  barometer  to  observe 
only  the  position  of  the  top  of  the  column 
and  apply  a  "correction  for  capacity." 
(See  paragraph  24.) 

19.  Contracted  barometer  scale. — As  the 
correction  for  capacity  in  barometers  with 
fixed  cisterns  remains  the  same  so  long 
as  the  quantity  of  mercury  within  the 
barometer  and  the  inside  area  of  tube 
and  cistern  are  unchanged,  it  will  not  be 
necessary  to  apply  a  capacity  correction 
to  every  reading  made,  provided  we  use 
on  the  barometer  a  scaie  having  all  its 
divisions  shortened  by  just  the  proper 
amount  to  compensate  for  the  capacity 
effect.  To  understand  this  more  clearly, 
imagine  a  barometer  with  the  top  of  the 
column  just  30  inches  above  the  surface 
of  the  mercury  in  the  cistern.  Suppose 
the  sectional  area  of  the  barometer  tube 
at  the  top  is  only  one-fiftieth  as  great  as 
that  of  the  cistern  (this  is  about  the 
usual  relation) .  Now  if  we  imagine  the 
column  to  rise  a  distance  of  1  inch  in  the 
tube,  it  will  then  seem  to  become  31 

inches  high,  but  when  the  column  rises  1  inch,  the  mercury  in  tho 
cistern  falls  one-fiftieth  of  an  inch,  and,  therefore,  the  real  height  of 


Hr* 

FIG.  9.  -Tuch  barometer  cistern. 


BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        17 


the  column  must  be  31^V  inches;  that  is,  we  may  say,  that  each  inch 
of  a  scale  represents  l^V  inches  of  change  in  the  real  height  of  the 
mercurial  column.  If,  therefore,  a  special  scale  be  prepared  having  the 
spaces  representing  inches,  each  one  fifty-first  part  of  an  inch  shorter 
than  a  true  inch,  then  readings  of  our  imaginary  barometer  on  such 
a  scale  will  indicate  the  true  height  of  the  column,  presupposing  of 
course  that  the  sectional  areas  of  the  tube  and  cistern  are  uniform, 
and  that  the  scale  is  adjusted|to  a  proper  distance  from  the  cistern. 

20.  By  methods  of  calibration  man- 
ufacturers are  able  to  construct  scales 
and    barometers    of    great    accuracy 
in  accordance  with  the  above  prin- 
ciples, and  they  are  very  convenient 
to  use. 

21.  It  is  obvious  that  if  a  barometer 
tube  in  such  an  instrument  is  broken 
it  will  be  difficult  to  find  another  so 
nearly  the  same  size  that  it  could  be 
used  with  the  old  scale;  generally  a 
new  scale  is  also  required. 

22.  In  figure  10  is  shown  a  cut  of  an 
excellent  barometer  of  the  fixed-cis- 
tern type,  devised  by  Schneider  Bros., 
of  Jersey  City,  N.  J.    One  of  the  special 
features  of  the  barometer  is  the  means 
provided   for  filling  the  cistern  and 
tube  with  mercury  so  that  the  barom- 
eter can  be  shipped  safely  from  place 
to  place. 

To  fill  the  cistern  with  mercury, 
the  barometer  is  first  very  carefully 
and  gradually  inclined  and  inverted. 
When  fully  inverted  the  mercury  suf- 
fices to  fill  up  the  cistern  just  to  the 
throat  of  the  contracted  portion  at  G. 
By  screwing  up  the  milled  head,  H, 
the  plate,  G,  closes  against  the  bottom 
of  the  cistern  and  completely  im- 
prisons the  mercury  with  only  a  little  free  space  for  expansion.  If  the 
barometer  is  now  turned  erect  the  mercurial  column  can  not  descend 
unless  the  screw,  H,  is  loosened,  whereupon  the  mercury  flows  into  the 
previously  unoccupied  space  below  the  plate,  G,  and  permits  the 
column  to  resume  its  normal  level. 

23.  The  capacity  correction  required  for  a  barometer  with  fixed 
cistern  and  true  scale  graduated  in  standard  units,  and  not  contracted, 

22937—12 2 


Fxo.  10.— Fixed  cistern  barometer. 


18        BAROMETERS    AND   MEASUREMENT   OF   ATMOSPHERIC    PRESSURE. 

as  explained  in  paragraph  19,  can  be  determined  from  careful  meas- 
urements of  the  internal  diameters  of  the  tube  and  cistern  made 
before  the  barometer  is  filled,  but  the  accuracy  of  this  correction 
should  always  be  checked  by  subsequent  comparisons,  as  indicated 
below. 

24.  Capacity  correction,  how  found. — It  is  evident  that  by  sliding  the 
scale  of  a  fixed-cistern  barometer  up  or  down  it  can  be  so  adjusted 
that  a  reading  at  some  one  point  is  just  right ;  for  example,  we  may 
place  the  30-inch  mark  so  that  when  the  top  ot  the  column  is  at  this 
mark  the  surface  of  the  mercury  in  the  cistern  is  just  30  inches 
below.     If  the  sectional  area  of  the  tube  is  a,  and  that  of  the  cistern 
A,  then,  if  the  mercurial  column  in  the  tube  rises  one  scale  division, 

the  fall  in  the  cistern  will  be  only  the  *  part  of  one  division.  That 
is,  the  correction  for  a  scale  reading  just  one  division  above  the  30- 
inch  mark  is :  +  ^  divisions ;  for  a  reading  two  divisions  above  the 

correction  is :  +  2  -j ,  etc.  This,  expressed  in  a  mathematical  formula, 
becomes — 

(Correction  =  C=  (h  —  R0)-*; 

^ 

in  which  R0  is  the  reading  at  which  the  correction  is  zero  and  h  is  the 
observed  reading,  uncorrected  for  temperature.  This  may  be  reduced 
to  the  following  simpler  form — 

C=nh  —  m; 

in  which  m  and  n  are  two  quantities  whose  values  are  best  determined 
from  a  complete  series  of  readings  of  the  actual  height  of  the  mer- 
curial column,  as  compared  with  the  reading  of  the  top  of  the  column 
of  the  fixed-cistern  barometer.  As  the  level  of  the  mercury  in  the 
cistern  is  generally  not  visible  in  barometers  of  this  type,  the  direct 
measurements  of  the  heights  of  the  column  can  not  be  made,  and  the 
necessary  actual  heights  must,  therefore,  be  obtained  from  readings 
of  some  standard  barometer. 

To  determine  the  values  of  m  and  n  accurately  by  comparisons, 
observations  should  be  made  over  a  greater  range  of  pressures  than 
ordinarily  occur  from  day  to  day,  and  the  best  results  will  require 
observations  under  pressure  artificially  changed  to  suit. 

25.  Changes  of  temperature  may  cause  the  sectional  areas  of  the 
tube  and  cistern  to  have  a  different  relation  than  that  assumed  in 
the  formula  above,  and  may  also  change  the  value  of  R0,  but  these 
effects  are  slight  and  are  not  considered  here. 


BAROMETERS   AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        19 

26.  Mercurial  barometer  for  marine  use. — Mercurial  barometers  con- 
structed upon  the  For  tin  system  and  other  forms  such  as  described  in 
the  preceding  paragraphs  are  almost  universally  used  on  land  for  the 
measurement  of  atmospheric  pressure,  and  no  other  form  of  barometer 
affords  as  great  accuracy  in  the  measurement  of  that  pressure.     If, 
however,  such  an  instrument  were  placed  aboard  ship  at  sea  the 
column  of  mercury  would  surge  up  and  down  the  tube  more  or  less 
violently  with  every  motion  of  the  vessel,  and  readings  would  be 
rendered  inaccurate  or  impossible. 

27.  This  difficulty  has  been  overcome  in  a  form  of  instrument 
known  as  the  Kew,  or  marine,  barometer.     Its  distinguishing  char- 
acteristics   consist    in    substituting    for    the    simple    straight    tube 
of  uniform  bore  commonly  employed  in  land  barometers,  a  tube 
having  a   wide  bore  for  6  or  8  inches  of  the  upper  portion  only. 
Below  this  the  tube  has  thick  walls  with  a  small  capillary  bore  only  a 
few  hundred ths  inch  in  diameter.     Near  the  bottom  end  the  bore  of 
the  tube  is  again  enlarged  to  form  an  air  trap,  all  as  shown  in  figure  11. 
If  small  quantities  of  air  chance  to  enter  the  open  end  of  the  tube  tlicv 
arc  not  likely  to    enter  the  small  point  of  the  inner  tube,  but  lodge 
instead  in  the  surrounding  space,  as  indicated,  where  the  air  must 
remain  and  does  not  affect  the  barometric  readings.     It  may  even  be 
removed  from  the  trap  when  the  barometer  is  undergoing  repairs. 

28.  The  flow  of  mercury  through  the  capillary  bore  takes  place  so 
slowly  that  the  column  can  not  surge  up  and  down  the  tube  seriously 
with  the  relatively  quick  motions  of  the  ship.     At  the  same  time  the 
height  of  the  column  adjusts  itself  to  the  slow  changes  of  atmos- 
pheric pressure,  and  thus  more  or  less  perfectly  answers  the  desired 
objects. 

•_".).  Figure  12  shows  a  high  grade  mercurial  barometer  a<laptc<l  to 
all  the  requirements  of  marine  use,  together  with  a  special  gimbal  sup- 
porting bracket  and  small  wooden  box,  into  which  the  barometer  and 
bracket  are  folded  and  thoroughly  protected  and  secured  when  not 
in  use. 

30.  The  glass  tube  and  boxwood  cistern,  all  as  shown  in  figure  11, 
are  secured  inside  the  bronze-metal  jacket  provided  at  the  top  with 
a  long,  slotted  opening  through  which  the  top  of  the  glass  tube  and 
mercurial  column  can  be  seen.     A  scale  of  graduations  is  fixed  beside 
the  opening  and  a  vernier  of  a  form  already  described  in  paragraph  12 
i-  arranged  to  slide  up  and  down  so  as  to  enable  accurate  mea-uic- 
ments  of  the  height  of  the  mercurial  column  to  be  made. 

31.  For  marine  use  it  is  necessary  that  the  barometer  be  live  to 
han.u:  in  a  vertical  line  despite  the  rolling  and  pitching  of  the  vessel. 
For  this  purpose  the  well-known   arrangement  of  irimbal   rin<r-   i- 
formed  upon  the  outer  extremity  of  a  hinged  bracket  and  >ecmed  to 
the  barometer  at  a  point  some  inches  above  the  middle. 


20        BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 


ill 


FIG.  11.— Tube  and  cis- 
tern of  marine  ba- 
rometer. 


FIG.  12. — Marine  barometer  and  box. 


BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        21 

In  the  position  shown  in  the  picture  the  barometer  is  readv  for 
reading,  and  the  tube  will  swing  on  the  gimbals  so  as  to  remain 
nearly  or  quite  vertical.  After  a  reading  has  been  taken  the  barome- 
ter must  not  be  left  exposed,  as  it  is  very  liable  to  injury  by  violent 
oscillations  in  heavy  weather.  In  the  equipment  of  the  standard 
Weather  Bureau  design  the  whole  bracket  barometer  and  all  are 
arranged  to  fold  up  compactly  within  the  small  mahogany  case,  the 
lid  of  which  closes  with  a  spring  clasp,  and  not  only  secures  the  barom- 
eter from  accidental  damage  but  from  undue  exposure  to  atmos- 
pheric- influences  as  well. 

32.  How  barometer  tubes  may  be  filled. — Processes  that  may  be  fol- 
lowed in  filling  barometer  tubes  for  high-grade  instruments  are  so 
rarely  described  and  so  little  known  that  a  short  description  of  some 
methods  frequently  employed  at  the  Weather  Bureau  with  highly 
satisfactory  results  will  doubtless   be  of   interest  to  a  number  of 
readers. 

The  object  of  any  filling  process  is  simply  to  introduce  pure  mer- 
cury and  totally  exclude  all  air,  moisture,  or  other  foreign  matter, 
especially  of  a  gaseous  nature,  that  may  possibly  later  occupy  the 
barometric  vacuum  and  cause  errors  by  the  pressure  it  exerts  upon 
the  top  of  the  mercurial  column. 

33.  Funnel  method. — It  may  seem  that  the  desired  result  could  be 
obtained   by  carefully  introducing   clean   mercury  through  a  long 
slender-stemmed  funnel  reaching  quite  to  the  closed  end  of  the  tube. 
(See  fig.  13.)     A  suitable  funnel  may  easily  be  made  by  drawing 
down  the  end  of  a  short  piece  of  rather  wide  glass  tubing.     Such  a 
method  is  sometimes  used  and  will,  indeed,  give  approximate  results, 
but  it  will  be  found  upon  investigation  that  while  the  mercury  seems 
to  drive  out  all  the  air,  yet  a  good  deal  will  still  be  found  in  the 
vacuum.     Originally,  this  air  mixed  with  water  vapor  is  strongly 
adherent  to  the  walls  of  the  glass  tube  by  reason  of  a  peculiar  prop- 
erty this  character  of  glass  is  found  to  have.     When  the  barometric 
vacuum  is  formed,  some  of  the  gaseous  matter  thus  attached  to  the 
tube  is  liberated  and  by  its  pressure  depresses  the  mercurial  column 
several  hundredths  of  an  inch,  as  has  been  shown  by  careful  experi- 
ment-. 

In  all  cases  it  is  of  great  importance  that  the  inside  walls  of  barom- 
ter  tubes  be  perfectly  clean.  New  tubes  are  thoroughly  cleaned 
with  whiting  or  other  suitable  material  while  open  at  both  ends,  and 
while  still  warm  and  dry  the  top  end  is  closed  and  the  cistern  end 
tapered  and  finally  fused  shut. 

Small  tubes  (one-quarter  inch  and  less)  that  have  become  soiled  by 
use,  exposure,  etc.,  can  not  be  easily  cleaned  properly,  and  such  are 
never  used  a  second  time  in  the  Weather  Bureau  work.  The  methods 


22        BAEOMETEES   AND   MEASUEEMENT   OF   ATMOSPHEEIC    PEESSUEE. 

given  in  paragraph  35  for  cleaning  larger  tubes  may,  however,  be 
used  even  with  these. 

One  very  simple  and  excellent  method  of  driving  off  nearly  all  the 
air  and  moisture  condensed  on  the  glass  walls  is  given  in  the  next 
paragraph. 

34.  The   boiling   method. — This    is    a    simple   method    commonly 
employed  with  all  small  tubes,  say,  one-fourth  inch  diameter,  more 
or  less;  such,  for  example,  as  is  required  in  the  several  types  of 
barometers  that  have  been  heretofore  described.     Much  larger  tubes 
are  frequently  boiled,  but  these  when  full  of  hot  mercury  are  diffi- 
cult to  handle,  a  strong  heat  is  required,  and  the  danger  of  serious 
accidents  is  considerable. 

It  is  well  at  first  to  warm  more  or  less  the  whole  tube,  and  the  cup 
of  clean  filtered  mercury  r  from  which  the  supply  is  drawn  should 
also  be  gently  warmed. 

Sufficient  mercury  to  fill  the  tube  3  or  4  inches  is  introduced  by  the 
aid  of  a  funnel  such  as  shown  in  figure  13,  except  that  the  slender  stem 
need  be  only  2  or  3  inches  long.  In  the  absence  of  such  a  funnel  it  is 
quite  as  well  to  employ  a  small  paper  cone  of  the  kind  commonly 
used  in  filtering  mercury.  The  mercury  in  the  tube  is  then  boiled 
carefully  over  a  good  Bunsen  burner  flame.  (See  fig.  14.)  For  this 
purpose  the  tube  is  held  easily  in  the  hands  and  moved  continuously 
through  the  flame  and  rotated  so  as  to  avoid  undue  local  heating  of 
the  tube.  As  the  heating  proceeds  the  air  and  moisture  vapor  first 
form  in  minute  silvery-white  bubbles,  giving  the  tube  a  frosted  ap- 
pearance. These  enlarge,  and  after  actually  boiling  the  mercury  for 
a  while  all  evidence  of  formation  of  bubbles  on  the  walls  disappears, 
and  further  boiling  of  the  mercury  takes  place  with  sudden  bursts 
and  with  sharp  metallic  clicks  as  the  portions  of  the  boiling  mercury 
strike  each  other  or  the  walls  of  the  tube.  When  it  is  apparent  that 
the  gases  on  the  walls  of  the  tube  have  been  driven  off  sufficiently,  a 
fresh  quantity — 3  or  4  inches — of  warm  mercury  is  added,  and  this 
portion  then  heated  and  boiled.  The  line  of  separation  between  the 
new  and  the  old  mercury  is  rendered  plainly  conspicuous  by  the 
frosted  appearance  previously  mentioned. 

These  operations  are  repeated  until  the  mercury  reaches  3  or  4 
inches  from  the  tip  of  the  tube,  the  latter  portion  being  filled  by  the 
careful  use  of  the  funnel  without  boiling.  If  the  walls  of  the  tube 
are  clean  and  dry  the  boiling  method  is  easy  to  employ  and  gives 
very  high  vacua. 

35.  Cleaning  large  tubes. — Tubes  that  have  contained  mercury  of 
which  oxidized  and  impure  portions  may  still  adhere  more  or  less 
closely  to  the  wall  should  first  be  treated  with  dilute  nitric  acid  (1 
part  in   20),    and   then   thoroughly   rinsed   with   plenty   of   water. 

i  For  notes  and  remarks  on  testing  and  filtering  mercury,  see  pars.  137  and  138. 


BAROMETERS   AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        23 

Ammonia  or  some  other  alkali  may  be  added,  if  desired,  after  which 
the  operations  described  below  should  be  followed. 

Introduce  several  inches  of  soapy  water  and  whiting  with  tissue- 
paper  pulp.  It  is  often  easiest  to  put  into  the  tube  several  small 
sheets  of  cheap  straw  or  manila  tissue  paper,  and  add  the  water  and 
whiting  afterwards. 


FIG.  13.  FIG.  14. 

Funnel  tube  and  boiling  method  of  filling  barometer  tubes. 

This  creamy  mass  can  be  strongly  shaken  about  inside  the  tube 
and  serves  to  scour  and  clean  the  walls  in  a  very  satisfactory  manner. 
It  is  then  removed  by  copious  rinsing  with  clean  water  and  after- 
war<U  with  distilled  water.  After  draining  some  minutes,  strong 
alcohol  in  moderate  quantities  is  several  times  introduced  and  suc- 
cessively drained  out  and  the  tube  given  a  final  draining  for  half  an 


24        BAROMETERS   AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

hour  or  so,  if  convenient,  after  which  it  is  ready  for  drying  and 
filling. 

36.  Air-pump  method  of  filling. — This  method,  with  numerous  modi- 
fications, has  been  employed  by  the  writer  in  a  large  number  of  cases 
with  very  satisfactory  results.  The  method  requires  a  good  air 
pump,  drying  tubes,  beakers,  burners,  stands,  etc.,  such  as  are  gen- 
erally available  in  any  physical  laboratory. 


FIG.  15. — Air-pump  method  of  filling. 

The  apparatus  is  arranged  as  shown  in  figure  15.  The  exhaustion 
and  funnel  tube  Ff  will  probably  require  to  be  made  up  to  suit  require- 
ments by  some  one  a  little  familiar  with  simple  glass-blowing  opera- 
tions. For  most  purposes  this  may  be  attached  to  the  barometer 
tube  by  a  short  piece  of  soft  pure  rubber  tubing.  The  outside  end 
of  the  funnel  is  drawn  down  into  a  long  capillary  extension  which  is 


BAROMETERS    AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        25 

bent  several  times,  as  shown,  so  as  to  dip  into  the  cup  of  mercury.  M. 
Too  fine  a  capillary  should  be  avoided,  and  it  is  generally  necessary 
and  easy  to  weaken  the  capillary,  as  at  o,  by  heating  it  a  little,  so  that 
later  the  tube  will  break  off  at  this  point  when  a  torsinal  strain  is 
put  upon  it  by  twisting  the  bent  extremity,  a  b  c.  The  point  at  c  is 
closed  by  fusion,  to  begin  with.  A  stopcock  may  be  employed,  as 
at  dj  and  in  this  case  the  breaking  of  the  tube  is  not  required,  but  if 
the  stopcock  leaks  even  a  little,  the  result  may  be  defective  and  the 
arrangement  first  described  is  often  best. 

37.  Drying  and  filling  the  tube. — In  order  to  dry  the  tube  it  is  alter- 
nately nearly  exhausted  and  dry  air  admitted  while  the  walls  are 
more  or  less  continuously  heated  by  playing  over  the  tube  with  the 
ilume  from  a  Bunsen  burner.     These  operations  must  be  repeated  10 
or  more  times  and  the  tube  kept  hot.     Throughout  these  operations 
the  mercury  is  excluded  and  the  funnel  tube  partakes  at  least  partly 
in  the  drying  influences.     While  the  tube  is  kept  quite  warm  and 
the  vacuum  maintained  at  a  high  point  the  capillary  is  broken  at 
a  and  the  mercury  in  M,  which  has  been  heated  in  the  meantime, 
is  permitted  to  flow.     M  need  not  be  large  enough  to  contain  all 
the  mercury  required,  but  additions  may  be  frequently  made  and 
the  whole  kept  quite  warm.     The  filling  will  take  place  slowly,  de- 
pending upon  the  size  of  the  bore  of  the  inlet  tube.     The  vacuum 
must  be  maintained  at  a  high  point  until  the  mercury  fills  the  barome- 
ter tube,  when  the  flow  may  be  stopped  by  admitting  air  to  the  pump. 
The  vessel  M  must  also  be  removed  if  there  is  any  tendency  of  the 
mercury  to  flow  one  way  or  the  other  by  gra  vitiation. 

For  the  very  finest  effects  the  barometer  tube  can  be  exhausted 
by  a  Sprengel  or  other  high  vacuum  pump,  but  in  this  case  the  rubber 
tube  connections  must  be  replaced  by  glass  and  tused  joints. 

38.  It  may  be  remarked  here  that  the  very  high  vacua  with  which 
we  are  familiar  nowadays,  in  X  ray  and  other  such  tubes,  are  by  no 
means  essential,  except  in  the  highest  grade  of  "normal"  barometers 
(see  paragraph  45),  where  results  depend  upon  the  absolute  height 
of  the  mercurial  column.     In  the  case  of  instruments  in  which  a  cor- 
rection is  found  by  comparison  with  a  normal,  and  especially  in  baro- 
graphs where  the  results  depend  entirely  upon  differences  in  the  posi- 
tion of  the  mercurial  column,  simple  methods  of  filling  give  entirely 
satisfactory  results.     In   these  cases  the  gaseous  pressure  in   the 
vacuum  is  so  nearly  constant  that  no  serious  error  is  involved. 

Suppose,  for  example,  that  the  residual  air  in  a  barograph  tube 
exerts  a  pressure  of  0.1  inch,  which  would  be  inexcusably  bad  fill- 
ing. Now,  since  we  set  the  pen  of  the  barograph  to  agree,  from  time 
to  time,  with  a  standard  barometer,  the  only  effect  the  air  can  have 
is  such  as  results  from  changes  in  pressure  due  either  to  changes 
in  temperature  or  changes  in  the  volume  of  the  vacuum  chamber. 


26        BAROMETERS    AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

A  20°  change  of  temperature  between  settings  of  the  pen  is  no 
usual  but  would  introduce  an  error  in  this  case  of  only  about  0.003 
inch,  whence,  with  reasonably  good  filling,  the  errors  from  imperfect 
vacua  are  entirely  insignificant. 

39.  Errors  of  barometers. — No  matter  how  carefully  a  barometer 
may  be  made,  certain  errors  due  to  various  causes  can  hardly  be 
eliminated.     In  the  first  place,  if  any  residual  air  or  vapor  or  any 
kind  of  gaseous  matter  remains  in  the  top  of  the  barometer  tube, 
the  column  of  mercury  will  not  rise  as  high  as  it  should.     We  know, 
likewise,  from  pnysical  laws,  that  the  capillary  forces  acting  between 
the  free  surface  of  mercury  and  the  glass  walls  at  the  top  of  the 
column  also  operate  to  prevent  the  mercury  from  rising  as  high  as 
it  should  in  the  tube.     Still  other  errors  arise  from  faults  in  the 
graduation  of  the  scale  and  from  failure  to  place  it  and  the  vernier 
at  exactly  the  positions  they  should  occupy. 

It  is  not  practicable,  or  necessary,  as  a  rule,  to  determine  these 
errors  separately.  When  an  instrument  is  completed,  its  readings 
are  carefully  compared  with  those  of  a  standard  barometer.  The 
differences  found  in  this  way  represent  the  outstanding  effect  of  the 
several  sources  of  error  mentioned  above  and  are  commonly  called 
the  "correction  for  instrumental  error  and  capillarity." 

40.  There  is  still  another  source  of  considerable  variation  in  the 
readings  of  mercurial  barometers,  namely,  the  influence  of  temper- 
ature, a  rise  of  temperature  expanding  both  the  metal  scale  and  the 
mercurial  column.     If  both  mercury  and  scale  expanded  equally  no 
correction  would  be  necessary,  but  the  mercury  expands  much  more 
than  the  scale,  so  that  a  large  correction  is  required  for  temperature. 

41.  The  following  detailed  discussion  of  the  several  errors  men- 
tioned above  will  make  the  matter  more  clear. 

(1)  Corrections  for  capillarity. — In  all  barometers  having  compara- 
tively small  tubes,  that  is,  of  less  diameter  than  from  0.7  to  1  inch,  the 
top  of  the  mercurial  column,  or  the  meniscus,  as  the  rounded  surface 
is  generally  called,  will  nearly  always  be  quite  convex  on  account  of 
the  capillary  action  between  the  mercury  and  the  glass.  In  conse- 
quence of  this  the  mercury  column  is  actually  depressed  a  slight 
amount  and  never  indicates  the  true  barometric  height.  This  source 
of  error  is  one  of  the  most  troublesome  to  which  barometers  are  sub- 
ject, as  the  capillarity  is  never  quite  constant  and  there  is  no  prac- 
ticable method  by  which  its  changing  value  can  be  accurately  deter- 
mined in  the  daily  use  of  an  instrument. 

The  error  due  to  capillarity  is  nearly  always  eliminated  as  far  as 
possible  from  the  scale  reading  by  adjusting  the  scale  so  that  allow- 
ance will  be  made  for  the  average  capillary  depression.  If  an  ordi- 
nary barometer  be  carefully  examined  it  will  be  found  that  the  30- 
inch  mark  on  the  scale  is  appreciably  less  than  30  inches  from  the 


BAROMETERS   AND   MEASUREMENT   OF   ATMOSPHERIC    PRESSURE.        27 

ivory  point.  In  general,  the  difference  represents  the  amount  the 
mercurial  column  is  depressed  by  capillarity.  A  portion  of  a  barom- 
eter scale  is  shown  enlarged  at  v,  figure  2  (at  the  top  and  at  the  right) . 
The  index  line  at  v  is  made  accurately  30  inches  from  the  ivory  point , 
but  the  30-inch -line  on  the  scale  is  shown  set  slightly  below  to  oi 
the  capillary  depression. 

(2)  Correction  for  imperfect  vacuum. — It  is  generally  assumed  that 
the  space  in  a  barometer  tube  above  the  mercurial  column  is  a  perfect 
vacuum,  and  that  there  is  no  downward  pressure  upon  the  top  of  the 
column  of  mercury.     This,  however,  is  not  strictly  the  case  in  any 
instance,  and  often  an  appreciable  quantity  of  air  or  water  vapor  i- 
present.     Any  vapor  that  the  mercury  may  give  off  is,  of  conr-e. 
always  present.     This  latter,  however,  is  very  small  and  is  never 
considered  except  in  the  most  refined  investigations.     If,  therefore. 
anv  such  pressures  exist  upon  the  top  of  the  column  it  will  be  de- 
pressed, and  a  correction,  which  may  be  properly  called  correction 
for  imperfect  vacuum  or  reduction  to  perfect  vacuum,  should  be  ap- 
plied.    Such  a  correction  will  vary  with  both  the  temperature  ami 
the  volume  of  the  space.     If  the  trace  of  air  present  is  slight,  •*  ifl 
nearlv  always  the  case  in  any  good  barometer,  the  correction  for 
vacuum  will  be  nearly  constant,  provided  the  volume  is  not  changed 
much  by  great  changes  of  pressure,  as  the  changes  corresponding  to 
ordinarv  changes  in  temperature  are  comparatively  small.     There- 
fore, in  ordinary  observatory  barometers  this  correction,  like  the  one 
for  capillarity,  is  included  in  the  correction  for  instrumental  error-. 
When,  however,  a  barometer  is  used  at  both  high  and  low  pressures. 
the  volume  of  the  vacuum  space  may  change  many  fold,  and  in  srn-h 
a  case  any  error  due  to  imperfect  vacuum  is  far  from  being  constant . 

(3)  Correction  for  instrumental  or  scale  error. — Errors  arising  from 
several  independent  sources  are  embraced  under  this  designation,  as, 
for  example:   (a)  The  graduated  scale  may  not  be  adjusted  so  per- 
fectly that  its  divisions  are  at  exactly  .the  right  distance  from  the 
ivory  point :   (6)  the  sighting  edge  of  the  vernier  may  not  be  true  of 
in    proper   correspondence   with   its   zero   graduation   line;    (c)    the 
unavoidable  errors  and  irregularities  hi  the  graduations  of  the  barom- 
eter scale   itself  also  introduce  different  errors  from  point  to  point 
along  the  scale.     Nevertheless,  sufficient  precision  in  scale  gradua- 
tion is  easily  attainable  even  in  a  scale  that  is  only  fairly  good,  and 
sueh  errors  are  generally  so  small  as  to  be  unimportant  in  ordinary 
barometric  observations  and  are  seldom  considered. 

As  stated  in  a  previous  paragraph,  the  combined  effect  of  such 
sources  of  error  as  those  just  mentioned  gives  rise  to  what  is  generally 
called  the  correction  for  instrumental  or  scale  error. 

The  manufacturer,  in  adjusting  a  good  barometer,  endeavor-  to 
eliminate  as  completely  as  possible,  or  at  least  to  reduce  to  a  very 


28        BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE. 

small  quantity,  the  several  corrections  mentioned  above,  viz,  cor- 
rection for  capillarity,  for  imperfect  vacuum,  and. for  instrumental 
error.  This  he  can  do  by  sliding  the  scale  up  or  down  a  small  frac- 
tion of  an  inch  until  he  finds  by  repeated  trials  and  comparative 
readings  with  a  standard  instrument  that  the  new  barometer,  when 
corrected  for  temperature,  as  described  below,  gives  the  same  or 
nearly  the  same  readings  as  the  standard.  Any  slight  outstanding 
difference  that  may  finally  remain  then  becomes  the  "  correction  for 
instrumental  error,  including  capillarity/'  or  briefly,  " correction  for 
scale  errors  and  capillarity.'' 

By  comparing  a  barometer  in  a  partial  vacuum,  so  as  to  ascertain 
the  " correction  for  scale  errors  and  capillarity"  at  several  pressures, 
such,  for  example,  as  at  each  inch  between  20  and  30  inches  of 
pressure,  it  has  been  learned  in  a  few  interesting  cases  that  very 
great  differences  in  the  correction  may  be  found  at  different  points 
of  the  scale.  These  differences  amounted  in  one  case  to  eighty- three 
thousandths  of  an  inch  between  25  and  30  inches,  and  could  not  be 
explained  by  any  error  of  the  scale  or  by  any  influence  other  than  that 
of  the  irregular  capillary  action  at  different  points  of  the  tube.  These 
investigations  demonstrated  the  necessity  of  ascertaining  the  correc- 
tion for  scale  errors  and  capillarity  of  each  instrument  for  the  par- 
ticular pressure  at  which  that  instrument  is  to  be  used. 

(4)  Correction  for  temperature. — The  temperature  of  a  barometer 
affects  the  accuracy  of  its  readings  in  two  ways.  First,  the  metal 
scale  expands  and  contracts  with  changing  temperatures,  and  is, 
therefore,  continually  changing  its  length.  Second,  the  mercury 
itself  expands  and  contracts  much  more  than  the  scale.  The  30 
cubic  inches  of  warm  mercury  in  a  barometer  tube  at,  say,  a  tempera- 
ture of  80°  F.,  will  be  more  than  1  ounce  lighter  than  the  same  volume 
of  mercury  at  the  freezing  temperature. 

The  true  pressure  of  the  air,  therefore,  is  not  shown  by  the  observed 
height  of  the  mercurial  column  until  we  take  into  account  both  the 
temperature  of  the  scale  and  the  density  of  the  mercury. 

For  this  reason  barometric  readings  require  to  be  reduced  to  a  read- 
ing which  would  have  been  obtained  had  the  mercury  and  scale  been 
at  certain  standard  temperatures. 

The  standard  temperature  adopted  for  the  mercury  is  always  that 
of  melting  ice—that  is,  0°  C.,  or  32°  F. 

When  the  readings  of  the  scale  are  taken  in  inches,  the  standard 
temperature  for  the  scale  reduction  is  then  62°  F.  If,  however,  the 
metric  unit  of  length  is  used,  the  standard  temperature  is  then  0°  C. 
In  the  latter  case  the  same  temperature  serves  for  both  the  scale 
and  the  mercury. 

There  is  thus  a  disparity  between  the  temperatures  at  which  Eng- 
lish and  metric  scales  are  of  standard  length;  moreover,  tables  of  baro- 


BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        29 

metric  corrections  for  temperature  usually  give  the  reduction  for 
both  the  scale  and  the  mercury  hi  one  correction,  whence  it  follows 
from  these  two  circumstances  that  the  corrections  in  English  and  in 
metric  tables  are  not  mutually  con  vert  able.  An  error  is  therefore 
introduced  if  the  unconnected  reading  of  a  mercurial  barometer 
expressed  in  metric  units  is  converted  into  English  units,  or  vice 
versa,  and  a  temperature  correction  afterwards  applied  to  the  result. 
The  conversion  of  barometer  readings  from  English  to  metric  or  from 
metric  to  English  units  can  only  be  made  correctly  after  each  read- 
ing has  been  fully  corrected  for  temperature.  A  further  discussion  of 
this  point  will  be  found  in  the  Monthly  Weather  Review  for  July, 
1898,  page  302. 

42.  Barometer  correction  cards. — Each  barometer  of  the  Weather 
Bureau,  when  sent  out,  is  accompanied  by  a  coirection  card  (Form- 

/No.  1059-Met'l)  showing  the  correction  for  instrumental  error,  and* 
also  the  corrections  of  the  attached  thermometer.  If  these  latter  correc- 
tions are  as  large  as  half  a  degree,  which  is,  however,  rarely  the  case, 
they  should  be  applied  to  the  reading  of  the  attached  thermometer 
before  taking  the  correction  for  temperature  from  the  table. 

43.  Tables  of  temperature  corrections. — Tables  of  correction  for  tem- 
perature are  computed  by  simple  formulae  taking  into  account  the 
known  coefficients  of  expansion  of  the  mercury  and  of  the  metal  or 
material  of  which  the  scale  is  made.     The  scale  in  this  sense  includes 
all  the  metal  parts  between  the  ivory  point  and  the  top  of  the  column 
of  mercury.     It  is  generally  assumed  that  the  temperatures  of  the 
scale  and  mercury  are  the  same,  and  that  the  temperature  is  given 
by  the  indications  of  the  attached  thermometer. 

For  barometers  with  brass  scales  the  following  formula  is  used  for 
computing  corrections: 

,.  „         -,       Z- 28.630 

-*1.1123«+ 10,978 

in  which  h  is  the  observed  reading  of  the  barometer  in  inches,  and  t 
is  the  temperature  of  the  mercury  and  scale  in  degrees  Fahrenheit. 

The  numerical  factors  in  this  equation  are  obtained  by  using  the 
folio  whig  values  for  the  expansion  of  mercury  and  brass,  viz: 

Cubical  expansion  of  mercury,  0.0001010  per  degree  Fahrenheit. 

Linear  expansion  of  brass,  0.0000102  per  degree  Fahrenheit. 

In  Section  VIII  are  given  full  tables  of  corrections  computed  by 
the  above  formula. 

44.  Correction  for  density  of  mercury. — If  the  density  of  the  mercury 
is  not  the  same  hi  two  barometers  that  are  exactly  alike  in  every  other 
respect,  the  heights  of  the  mercurial  columns  will  not  be  the  same 
for  the  same  pressure.     In  such  a  case  a  reduction  to  mercury  of  a 
standard  density  will  be  required.     The  presence  of  1  per  cent  of  lead 
with  mercury  causes  a  change  in  density  that  would  require  a  correc- 


30        BAROMETERS    AND    MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

tion  of  about  0.051  of  an  inch.  On  the  other  hand,  mercury  contain- 
ing even  so  little  as  one  one-hundredth  of  1  per  cent  of  lead  is  ren- 
dered so  exceedingly  foul  that  it  could  not  be  used  for  barometric 
purposes.  It  is  therefore  easily  seen  that  a  correction  for  standarc 
density  is  a  refinement  which  need  not  ordinarily  be  considered. 

NORMAL    BAROMETER — STANDARD    BAROMETER. 

45.  It  is  easily  understood,  after  what  has  been  said  above  aboul 
errors  of  graduation,  errors  due  to  capillarity,  to  imperfect  vacuum 
to  instrumental  imperfection,  etc.,  that  even  the  best  of  ordinary 
barometers  is  liable  to  be  quite  incorrect  until  corrections  for  these 
errors  have  been  determined.  Moreover,  from  the  nature  of  things 
we  can  not  determine  these  corrections  except  by  comparison  with  a 
standard  barometer,  and  the  question  might  properly  be  asked,  How 
do  we  know  the  standard  barometer  is  right?  We  will  answer  this 
by  saying  that  the  standard  barometer  ought  to  be  a  normal  barom- 
eter. So  few  understand  clearly  the  distinction  between  these  words 
" standard"  and  " normal7'  in  the  present  connection  that  somd 
explanation  is  necessary.  In  the  first  place,  the  expression  "  normal 
barometer"  is  used  a  great  deal  by  the  Weather  Bureau  and  meteorol- 
ogists in  general  when,  strictly  speaking,  the  expression  should  be 
normal  barometric  pressure;  by  which  is  meant  the  average  of  a 
great  many  years'  observations  of  atmospheric  pressure  at  a  single 
station.  In  the  present  case  the  word  " normal"  has  an  entirely 
different  meaning. 

A  standard  barometer  need  not  necessarily  be  anything  more  than 
an  instrument  which  has  been  pronounced  to  be  correct  by  some 
special  authority.  For  instance,  the  Congress  of  the  United  States 
might  say  that  the  indications  of  such  and  such  an  instrument 
represent  the  true  atmospheric  pressure  and  that  the  particular 
barometer  in  question  is  the  standard  of  the  nation.  Such  an  instru- 
ment, although  formally  pronounced  to  be  a  standard,  might,  never- 
theless, possess  little  more  than  the  average  accuracy  and  its  indica- 
tions still  be  more  or  less  erroneous.  Since  the  several  errors  to 
which  barometers  are  subject  can  not,  in  the  majority  of  cases,  be 
determined  except  by  comparison  with  an  instrument  whose  errors 
are  all  known,  a  standard  based  only  on  the  dictum  of  some  authority 
can  not  necessarily  be  regarded  as  giving  true  indications.  A  normal 
barometer,  however,  is  one  the  construction  of  which  is  such  that  the 
instrument,  fundamentally  and  independent  of  all  other  similar  instru- 
ments, gives  a  true  measure  of  the  pressure  of  the  air. 

Standard  barometers  should  therefore  generally  be  also  normal 
barometers.  It  must  not  be  understood  that  a  normal  barometer  is 
absolutely  without  any  error.  The  construction,  however,  is  such 
that  those  errors  which  can  not  be  wholly  eliminated  can  yet  be 
ascertained  from  the  indications  of  the  instrument  itself.  The  error 


BAROMETERS    AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        31 

for  capillary  action,  for  example,  is  wholly  eliminated  by  employing 
a  tube  of  very  large  diameter.  On  the  other  hand,  if  the  vacuum  is 
not  sufficiently  perfect,  the  error  from  this  cause  can  still  be  ascer- 
tained, for  the  barometer  will  be  constructed  so  that  readings  can  be 
made  when  the  vacuum  chamber  is  large,  and  again  when  it  is  many 
times  smaller  and  the  pressure  of  the  remnant  of  air  therein  propor- 
tionately increased;  from  such  readings  the  desired  corrections  can 
be  computed.  So,  also,  other  errors  are  either  eliminated  or  are 
ascertained  by  special  investigations,  and  the  reading  of  the  barometer 
after  all  known  corrections  are  made  is  regarded  as  fundamentally 
correct. 

Barometers  of  this  type  are  generally  elaborate  of  construction 
and  will  not  be  described  here.  Several  of  the  European  normals 
are  fully  described  by  Prof.  Abbe  in  the  Annual  Report  of  the  Chief 
Signal  Officer,  1887,  Part  II. 

II. ANEROID    BAROMETERS. 

46.  Figures  16  and  17  represent  two  of  the  more  important  types 
of  aneroid  or  holosteric  barometers,  showing,  principally,  the  internal 
mechanisms.     The  first  is  a  more  common  form,   but  the  second 
i-.  in  general,  somewhat  better.     The  essential  feature  is  the  same 
in  both  instruments  and  consists  of  the  small  metallic  box  or  cell, 
J/,  the  upper  and  lower  walls  of  which  are  made  of  very  thin  circular 
sheets  of  corrugated   German  silver,   which   are  soldered   together 
on   their  outer  edges,  forming  a  very  short  cylinder.     The  air  is 
thoroughly  exhausted  from  this  cell  through  a  tube  at  one  side, 
which,  when  the  vacuum  is  as  perfect  as  desired,  is  pinched  tightly 
together,  cut  off,  and  hermetically  sealed  with  solder,  producing  the 
projection  seen  at  c.     The  flexible  corrugated  surfaces,  which  tend  to 
be  collapsed  by  the  pressure  of  the  outside  air,  are  forcibly  held  apart 
by  the  action  of  a  strong  steel  spring,  R.     As  the  pressure  of  the  air 
increases  the  spring  is  compressed  and  the  corrugated  surfaces  ap- 
proach each  other  slightly,  returning  again  or  separating  still  farther 
with  diminution  of  pressure.     To  measure  the  changes  in  atmospheric 
pressure,  it  is  only  necessary  to  measure  the  minute  movement-  of 
this  flexible  cell. 

The  two  forms  of  aneroid  figured  herein  differ  simply  in  the  man- 
ner by  which  the  minute  alterations  in  the  elastic  yielding  of  the 
spring  are  magnified  and  rendered  measurable. 

47.  In  the  common  aneroid  a  lever,  Z,  attached  directly  to  the  spring 
connects  by  a  link,  ra,  with  a  very  short  arm  of  a  sort  of  bell-crank 
lever,  r,  t,  having  a  horizontal  axis  on  pivots  at  each  end. 

The  longer  arm,  t,  of  this  bell-crank  lever  is  connected  by  means  of 
a  wire,  s,  with  a  very  fine  chain,  the  other  end  of  which  winds  around 
a  -mall  wheel  or  drum  on  the  axis,  a,  upon  which  is  mounted  the  hand 


32        BAROMETEES   AND  MEASUREMENT   OF   ATMOSPHERIC    PRESSURE. 

as  seen.  At  6  is  shown  a  small  spiral  steel  spring,  like  the  hair- 
spring of  a  watch,  which  serves  to  take  up  the  slack  in  the  loose  con- 
nections of  the  numerous  joints,  levers,  and  links. 

At  r  is  shown,  also,  a  small  counterpoise  weight  attached  to  the 
bell-crank  lever  to  aid  in  securing  a  more  stable  position  of  the  index 
when  the  barometer  is  placed  in  different  positions:  that  is,  whether 
the  dial  is  horizontal,  or  vertical,  or  turned  to  one  side  or  the  other. 

The  point  of  attachment  of  the  link,  ra,  to  the  bell-crank  lever  is 
sometimes  adjustable  so  that  the  movements  of  the  hand  can  be  made 
to  correspond  to  the  value  of  the  scale  graduations. 

The  steel  spring,  R,  is  also  slightly  adjustable  by  means  of  a  screw 
from  the  underside  threaded  into  the  part,  N.  This  permits  adjusting 
the  hand  to  any  particular  point  of  the  scale  to  give  correct  readings. 


FIG.  16. — Aneroid  barometer. 


48.  Effects  of  temperature.— The  steel  spring  and  the  feebler  elastic 
reaction  of  the  composition  metal  of  the  vacuum  chamber  are  appre- 
ciably weakened  by  increase  of  temperature,  so  that  in  some  cases  j 
a  rise  of  the  pressure  may  seem  to  occur  which  is  really  caused  by 
the  weakening  of  the  spring.     In  some  cases  efforts  are  made  to  com- 
pensate for  this  by  leaving  a  small  quantity  of  air  in  the  vacuum 
chamber,  which  when  heated  increases  its  pressure  upward  and  tends  i 
to  offset  the  weakening  effect  upon  the  springs.     A  better  plan  is  to  I 
make  the  lever,  Z,  of  two  different  metals,  viz,  brass  and  iron,  firmly 
brazed  together.     The  differential  expansion  of  these  two  metals  with 
temperature  changes  produces  flexure  in  the  lever.     By  filing  and 
adjusting  the  bimetallic  bar,  the  flexure  due  to  temperature  can  be 
made  very  nearly  to  balance  the  effect  of  temperature  on  the  spring. 


BAROMETERS    AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        33 


The  aneroid  is  then  said  to  be  " compensated"  and  this  word  is  often 
found  on  the  dial.  In  many  cases  this  word  is  there  when  the  com- 
pensation is  very  imperfect. 

49.  Dejects. — The  friction  and  looseness  in  the  joints  of  the  links 
and  the  lack  of  perfect  balance  in  the  various  parts  give  rise  to  con- 
tinually changing  errors  hi  the  reading  of  the  aneroids.     This  will 
be  shown  by  tapping  the  aneroid  from  different  sides  and  holding  it 
in  a  variety  of  positions;  a  different  reading  will  be  given  for  each 
condition. 

50.  Goldschmidt's  aneroid. — The  numerous  levers  and  links  in  the 
common  aneroid  are  dispensed  with  in  this  form,  and  the  minute 
movements  of  the  cell  and 

spring  are  measured  di- 
rectly by  means  of  a  mi- 
crometer screw. 

This  is  accomplished  in 
several  different  ways  by 
manufacturers,  a  common 
form  of  instrument  being 
shown  in  figure  17,  where 
the  parts  have  been  sepa- 
rated for  a  better  view. 
The  plate,  B,  with  its  at- 
tached mechanisms,  is  se- 
cured in  the  bottom  of  the 
box,  A.  The  micrometer 
screw,  S,  works  through 
the  cover  of  the  box.  The 
corrugated  aneroid  vacu- 
um chamber,  M,  is  held 
distended  in  the  usual 
manner  by  the  steel 
spring,  R.  A  sharp  knife- 
edge  projection,  a,  of  a 
double-formed  lever,  I, 
rests  upon  a  smooth  pol- 
ished spot  near  the  outer  end  of  the  spring.  This  spot  is  some- 
tunes  a  bit  of  glass  or  agate.  The  lever,  Z,  is  pivoted  delicately 
upon  an  axis  at  r  and  is  formed  of  two  parts  joined  near  the  axis. 
The  upper  piece  of  this  lever  is  a  very  delicate  steel  spring,  with  a 
flat  polished  surface  at  o,  which  by  the  springiness  of  the  arm  presses 
against  the  point  of  tfie  micrometer  screw,  S'.  At  the  ends  the 
spring  and  lever  are  formed  with  little  flat  surfaces,  each  having  a 
fine  line  engraved  across  the  middle.  This  construction  is  not 
clearly  seen  in  the  drawing.  To  observe  the  air  pressure  the  aneroid 
22937—12 3 


FIG.  17.— Goldschmidt's  aneroid. 


34        BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE. 

must  be  "set"  by  bringing  the  above-mentioned  lines  into  coincidence. 
For  this  purpose,  and  at  the  same  time  to  measure  the  movement 
necessary  to  bring  about  such  a  coincidence,  the  finely  cut  microm- 
eter screw,  £',  is  provided.  The  large  head,  £,  having  a  scale  of 
graduations  engraved  upon  its  outer  rim,  being  turned,  the  point 
of  the  screw  presses  against  the  spring  at  o,  deflects  it  so  that  the 
lines  upon  the  ends  of  the  spring  and  lever  may  be  placed  in  exact 
coincidence.  To  facilitate  making  this  adjustment  accurately,  a 
small  magnifying  glass,  L,  is  generally  provided.  The  small  scale,  P, 
is  opposite  the  ends  of  the  lever,  Z,'when  the  mechanisms  are  in  their 
normal  position  and  indicates  the  whole  number  of  turns  made  by 
the  screw,  or,  what  is  the  same  thing  and  more  convenient,  shows 
the  pressure  corresponding  to  the  successive  positions  of  the  screw. 
The  fractions  of  a  turn  are  indicated  accurately  by  the  graduations 
on  the  head  of  the  screw. 

51.  Temperature  effects. — The  Goldschmidt  aneroid  is  not  compen- 
sated for  temperature,  but  is  generally  accompanied  by  a  table  of 
corrections  therefor,   the  temperature  being  indicated  by  a  small 
thermometer,  the  scale  of  which,  in  the  aneroid  shown,  is  visible  j 
through  an  opening  at  T. 

52.  Reading,  how  made. — Aneroids  of  this  pattern  are  read  by  first 
turning  the  micrometer  screw  until  the  lines  upon  the  spring  and  1 
lever  come  into  exact  coincidence.     The  reading  on  the  scale,  P,  is  j 
noted,  and  to  this  is  added  the  part  taken  from  the  graduations  on  j 
the  head. 

53.  Consult  paragraphs  104  to  106  for  information  respecting  the 
use  of  the  aneroid  in  determining  elevations. 

54.  How  adjusted  to  standard  pressures. — The  aneroid  barometer,  no  | 
matter   how  perfectly  constructed,  does  not  indicate  any  particular 
pressure  until  by  careful  comparison  with  a  standard  barometer  its 
index  is  adjusted  to  give  as  nearly  as  possible  the  same  reading  as 
the  standard.     This  adjustment  is  made  by  means  of  the  screw,  which  | 
in  nearly  all  aneroids  is  seen  just  within  a  small  hole  in  the  back  of  j 
the  case.     The  graduations  of  the  dial  must  of  course  be  such  as  to 
show  changes  of  pressure  on  a  scale  of  millimeters  or  inches. 

The  Goldschmidt  aneroid  is  similarly  adjusted  in  a  variety  of  ways,  ! 
of  which  a  common  one  is  to  shift  the  zero  or  index  line  at  which  the  j 
reading  of  the  micrometer  screw  is  made. 

55.  Errors  and  dejects  oj  aneroids  in  general. — Alter  being  once  j 
adjusted  to  give  accurate  pressures,  as  already  described,  the  aneroid  | 
should  be  handled  with  great  care.     Violent  knocks  and  shaking  i 
will,  especially  with  the  common  aneroid,  almost  certainly  change 
or  shift  the  various  links  and  levers  in  their  joints  and  change,  more 
or  less  permanently,  the  position  of  the  index.     For  such  reasons 
aneroids  are  very  liable  to  acquire  unknown  and  often  large  acci- 


BAROMETEES   AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        35 

dental  errors,  and  can  not,  therefore,  be  regarded  as  very  satisf actory 
instruments. 

56.  Errors  due  to  very  slow  changes,  "creeping." — If  an  aneroid 
adjusted  to  read  correctly  under  ordinary  air  pressures  is  placed 
within  the  receiver  of  an  air  pump,  the  index  will  quickly  fall  to  a 
lower  pressure  when  a  partial  vacuum  is  formed.  If,  however,  the 
vacuum  be  maintained  constantly  at  the  same  pressure  for  many 
days  in  succession,  the  reading  of  the  aneriod  will  be  found  grad- 
ually to  become  lower  and  lowrer,  but  after  three  or  four  weeks 
further  changes  cease  or  are  very  small.  The  amount  of  this  slow 
change  differs  greatly  and  may  be  from  one-half  inch  or  less  to  over 
an  inch,  according  to  the  diminution  of  pressure  and  other  circum- 
stances. Again,  when  the  barometer  is  removed  from  the  air  pump 
it  does  not  immediately  return  to  its  original  correct  reading,  but  its 
indications  will  be  found  to  be  too  low,  several  weeks  being  again 
consumed  in  a  slow  return  to  approximately  its  former  correct  reading. 

This  "creeping"  action  depends,  no  doubt,  upon  some  molecular 
changes,  as  yet  not  clearly  understood,  that  take  place  within  the 
materials  of  the  aneroid  box  and  steel  springs.  In  any  case  the  read- 
ings are  liable  to  be  very  seriously  in  error,  and  tourists  and  others 
who  carry  with  them  aneroids  for  the  purpose  of  ascertaining  eleva- 
tions should  have  means  to  determine  and  eliminate  the  very  serious 
errors  referred  to  above.  A  further  discussion  of  these  errors  will 
be  found  in  the  Monthly  Weather  Review  for  September,  1898, 
page  410. 

")7.  The  aneroid  barometer  is  a  convenient  instrument  for  showing 
more  or  less  accurately  the  character  and  the  amount  of  barometric 
cluniffps  going  on  from  day  to  day,  but  the  mercurial  barometer  is  the 
oniv  instrument  that  gives  atmospheric  pressures  with  that  degree 
of  precision  required  in  simultaneous  meteorological  observations. ' 

">s.  Test  of  condition  of  aneroid,  —  Aneroids,  seemingly  good,  are 
often  defective,  because  some  of  the  joints  of  the  levers  and  pivots 
are  too  tight,  causing  the  hand  to  stick  and  not  move  with  the  perfect 
freedom  it  should.  The  condition  of  an  aneroid  can  be  quickly  tested 
in  this  respect  by  tapping  the  instrument  on  the  side  or  bottom  with 
the  fingers  or  knuckles,  or  perhaps  better  by  lifting  the  instrument 
about  one-fourth  of  an  inch  from  a  table  or  cane-seated  chair  and 
placing  it  back  again  somewhat  sharply.  Under  this  treatment,  if 
the  joints  and  levers  are  perfectly  free,  the  hand  will  jump  away  and 
t  lieu  return  quickly  with  a  vibratory  movement  to  its  original  position. 
If  the  instrument  is  defective,  the  hand  in  some  cases  will  not  respond 
to  the  slight  knocks,  or  will  do  so  without  exhibiting  any  vibratory 
movement,  or  upon  being  disturbed  it  may  move  a  little,  but  will  not 
return  to  its  original  position. 


36        BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC   PRESSURE. 
m< MISCELLANEOUS    BAROMETERS. 

59.  Many  curious  and  interesting  forms  of  barometers  have  been 
devised  for  the  purpose  of  showing  the  changes  of  air  pressure  in  a 
much  more  magnified  manner  than  is  possible  with  ordinary  barome- 
ters, especially  of  the  mercurial  pattern.     A  few  of  these  will  be 
briefly  mentioned.     Many  others  equally  curious  and  meritorious  are 
necessarily  omitted. 

60.  Water  barometer. — This  may  be  constructed  in  practically  the 
same  manner  as  the  mercurial  barometer,  except  that  water  is  used 
instead  of  mercury.     Now,  as  mercury  is  13.6  times  as  heavy  as  water, 
it  will  result  that  the  water  column  will  be  13.6  times  higher  than  the 
mercurial  column,  or  about  34  feet  high;  also  a  change  of  1  inch  in 
pressure  by  the  mercurial  barometer  will  appear  as  a  change  of  13.6 
inches  in  the  water  barometer. 

The  great  defect  of  the  water  barometer,  aside  from  its  inconven- 
ient proportions,  is  the  shortening  of  the  column,  due  to  the  pressure 
of  water  vapor  in  the  vacuum.  This  shortening  amounts  to  about  10 
inches  at  a  temperature  of  70°  F.,  and  if  the  temperature  were  to  rise 
10°  the  pressure  would  seem  to  fall  nearly  4  inches,  when  really  no 
change  of  pressure  had  occurred. 

61.  Glycerin  barometer. — Glycerin,  sulphuric  acid,  or  nonvolatile 
oils  may  also  be  used  in  place  of  mercury,  each  affording  a  magnified 
indication  of  pressure  changes,  as  in  the  case  of  the  water  barometer, 
and  with  the  advantage  that  the  errors  due  to  vapor  pressures  are 
much  less,  or  are  quite  inappreciable. 

62.  Diagonal  barometers.— It  is  easily  seen  that  if  the  top  por- 
tion of  a  barometer  tube  be  bent  off  at  a  small  inclination  upward 
from  a  horizontal  position,  the  movement  of  the  mercury  along  this 
diagonal  portion  will  exhibit,  in  a  magnified  manner,  the  ordinary 
fluctuations  of  atmospheric  pressure.     The  same  result  is  secured  by 
forming  the  top  portion  of  a  barometer  tube  into  a  helical  coil  having 
suitable  graduations. 

63.  Dial  barometers. — Probably  the  most  practical  expedient  for 
magnifying  the  indications  of  an  ordinary  siphon  mercurial  barometer 
is  the  form  known  as  the  "dial  barometer/'  shown  in  figure  18. 

The  action  of  the  instrument  will  be  readily  understood  from  the 
diagram  without  further  explanation. 

64.  Sympiesometer. — This  is  another  form  of  sensitive  barometer, 
in  which  the  pressure  of  the  air,  acting  through  a  short  column  of 
liquid  of  low  specific  gravity,  is  made  to  compress  a  portion  of  air 
confined  within  the  instrument.     In  this  case  it  is  necessary  to  make 
allowance  for  the  expansion  and  contraction  of  the  confined  air  with 
changes  of  temperature.     This  is  conveniently  accomplished  by  pro- 
viding the  instrument  with  two  scales,  one  of  which,  containing  the 
readings  of  pressure,  is  set  to  a  certain  line  on  the  other  scale  accord- 


BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        37 


ing  to  the  temperature  indicated  by  the  attached  thermometer  of  the 
apparatus. 

65.  Howson's  barometer. — This  ingenious  and  novel  device  is  shown 
in  section  in  figure  20.     The  barometer  tube  is  large  in  diameter  and 
longer  than  usual.     The  cistern  is  recurved  and  extends  as  a  long  core 
up  inside  the  barometer  tube,  reaching  within  3  or  4  inches  of  the  top 
of  the  column  of  mercury.     The  proportions  are  arranged  to  be  such 
that  the  upward  pressure  of  the  air  on  the  under  side  of  the  cistern  is 
sufficient  to  sustain  the  cistern  and  contents  suspended  from  the  end 
of  the  barometer  tube. 

When  a  change  of  pressure  occurs  the  cistern 
is  caused  either  to  ascend  upon  a  barometer 
tube  or  to  move  down  to  a  new  position  of  equi- 
librium, and,  by  selecting  suitable  proportions 
between  the  internal  diameter  of  the  barometer 
tube  and  the  thickness  of  its  walls  in  relation  to 
the  diameters  of  the  core  and  cistern,  the  move- 
ment of  the  latter  up  and  down  the  tube  with 
changes  of  atmospheric  pressure  can  be  made  to 
represent  pressures  upon  a  greatly  magnified  scale. 

66.  Magnifying  siphon  barometer. — Figure  21 
illustrates  still  another  expedient  by  which  the 
indications  of  a  plain  mercurial  barometer  may 
be  greatly  magnified.     The  short  leg  of  a  siphon 
barometer  is  extended  upward,  in  the  manner 
shown.     The  small-bore  tube,  a,  is  enlarged  at 
the  top  to  have  the  same  diameter  as  the  cistern 
portion.     The  portions,  a,  6,  c,  of  the  short  leg 
are  filled  up  to  a  point  at  about  Ji  with  colored 
water,  and  then  above  this  with  kerosene  or  some 
other  liquid  of  nearly  the  same  density  as  water 
and  with  which  it  does  not  mix.     It  is  plain 
that  when  the  level  of  the  mercury  hi  the  short  leg 
changes,  the  meniscus  separating  the  oil  from  the 

water  in  the  small-bore  tube  will  be  seen  to  change  its  position  by 
an  amount  enlarged  in  proportion  to  the  relation  between  the  areas 
of  the  cistern  and  the  small  tube. 

67.  General  comments  on  magnifying  barometers. — The  special  barom- 
eters described  above  are  interesting  and  in  many  respects  curious, 
but  they  can  not  be  regarded  as  anything  more  than  philosophical 
toys  and  curiosities.     It  is  impracticable  in  any  of  the  magnifying 
instruments  to  determine  the  absolute  pressure  with  great  precision, 
for  the  expedients  of  magnification  introduce  sources  of  both  constant 
and  accidental  errors  that  affect  the  results  to  a  magnified  extent,  so 
that  even  less  precision  generally  results  than  is  attained  with  well- 
made  mercurial  barometers  of  the  simple  pattern. 


FIG.  18.— Dial  barometer. 


38        BAROMETERS   AND   MEASUREMENT    OF   ATMOSPHERIC   PRESSURE. 


Fro.  19,— Syrtipiesometf-r. 


FIG.  20.— Howson's 
barometer. 


FIG.  21.— Three-liquid 
barometer.   *fffl 


BAROMETERS    AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        39 
IV.    BAROGRAPHS,  OR   CONTINUOUSLY   RECORDING   BAROMETERS. 

68.  The  barograph  is  a  form  of  barometer  with  the  addition  of 
parts  by  which  a  continuous  record  of  the  barometric  oscillations  is 
traced  upon  a  sheet  of  moving  paper  or  obtained  by  photographic 
processes  upon  sensitized  plates.     Many  different  forms  of  apparatus 
have  been  devised,  nearly  all  of  which  are  more  or  less  elaborate,  in 
general,  but  not  correspondingly  accurate.     In  most  cases  the  chang- 
ing temperature  to  which  the  instruments  are  subjected  introduces 
small  errors,  and  the  mechanisms  effecting  continuous  registration 
either  obstruct  the  free  action  of  the  barometer  proper  or  do  not 
transmit  to  the  record  sheet  a  trace  representing  exactly  the  original 
fluctuations  of  pressure.     The  absolute  value  of  the  pressure  indi- 
cated by  the  position  of  the  tracing  point  is  also  subject  to  uncertain- 
ties, and  while  changes  of  pressure  are  shown  with  greater  or  less 
accuracy,  it  is  impossible,  even  with  the  best  instruments,  to  record 
the  absolute  pressure  with  a  precision  equal  to  that  of  an  eye  reading 
of  a  standard  barometer.     In  general,  therefore,  the  indications  of 
automatic  instruments  are  checked  and  corrected  by  reference  to  occa- 
sional eye  readings  of  a  standard  barometer. 

69.  It  hardly  requires  to  be  said  that  the  mercurial  barograph  is 
more  reliable  and  gives  more  accurate  results  than  those  of  the 
aneroid  type.     Many  ingenious  mechanisms  are  employed  to  effect 
the  continuous  registration,  thus: 

(1)  In  some  cases  this  is  accomplished  by  directly  photographing 
on  a  moving  sheet  of  sensitized  paper  or  a  moving  plate  the  changing 
positions  of  the  summit  of  a  barometric  column,  notably  the  case  at 
the  Kew  Observatory,  England. 

(2)  In  other  forms,  a  float  resting  upon  the  mercury  in  the  open  leg 
of  a  siphon  barometer  communicates  the  motion  directly  to  a  lever 
which  carries  the  recording  pen  at  its  end  and  produces  the  record  on 
a  magnified  scale.     It  is  difficult  to  overcome  the  friction  of  the  pen 
and  magnifying  levers  sufficiently  to  obtain  correct  records;  the  con- 
struction in  some  forms  is  therefore  modified  and  clock  movements  or 
electromagnets  are  caused  to  perform  the  real  labor  of  producing  the 
record.     Under  this  category  we  have  such  cases  as  follow. 

(3)  The  motion  of  the  float  is  communicated  to  a  very  delicately 
poised  lever,  the  slightest  movement  of  which  sends  a  current  of  elec- 
t  ririty  through  properly  disposed  magnets,  whiph  either  alone  or  act- 
ing in  conjunction  with  clockwork  perform  the  real  labor  of  moving 
the  pen  mechanisms  and  preserving  a  proper  condition  in  the  equilib- 
rium of  the  float. 

(4)  A  distinct  class  of  barographs  is  obtained  by  constructing 
mechanisms  which  measure  and  record  the  barometric  oscillations  by 
weighing  the  changing  quantities  of  mercury  within  a  poised  barome- 


40       BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE. 

ter  tube  or  cistern.  The  weighing,  is  effected  by  suspending  .the 
barometer  tube  or  its  cistern  from  the  beam  of  a  balance  which  is  con- 
tinuously preserved  in  a  condition  of  equilibrium  by  the  automatic 
movement  of  a  counterpoise  traveling  along  the  beam  of  the  balance. 
The  movement  of  the  counterpoise  is  effected  either  by  clockwork, 
or  by  electromagnets,  or  by  both. 

(4a)  A  modification  of  the  weighing  principle  consist  in  substi- 
tuting jx>r  the  balance  mechanisms,  coiled  steel  springs  by  the  deflec- 
tion of  which  the  changing  weights  are  measured  and  recorded. 

The  following  descriptions,  given  in  some  detail,  of  representative 
types  of  mercurial  barographs  that  have  been  maintained  in  opera- 
tion by  the  Weather  Bureau  will  enable  the  student  interested  to 
understand  more  fully  how  such  mechanisms  operate : 

70.  Foreman's  barograph. — This  is  shown  in  figure  22.  It  belongs 
to  the  class  mentioned  above  under  (3).  Prof.  G.  W.  Hough,  director 
of  Dudley  Observatory,  Albany,  N.  Y.,  about  1862,  perfected  baro- 
graphs recording  on  this  principle,  the  form  here  figured  being  designed 
by  Mr.  H.  L.  Foreman,  who  was  at  one  time  Prof.  Hough's  assistant, 
The  glass  siphon  tube  of  the  baragraph  is  at  the  back  at  B,  and  is 
only  partly  visible,  the  bend  being  hidden  behind  the  record  cylinder, 
A.  The  open  end  of  the  siphon  tube  is  seen  at  0;  an  iron  float  rests 
lightly  upon  the  surface  of  the  mercury  within,  being  sustained  by 
means  of  a  fine  wire,  t,  from  the  short  end  of  the  lever,  Z,  which  is  deli- 
cately poised  upon  steel  knife  edges  at  r.  The  long  end  of  this  lever 
at  h  is  tipped  with  platinum  and  placed  between  two  platinum- 
pointed  screws,  both  of  which  nearly,  but  not  quite,  touch  the  tip  of 
the  lever  when  the  latter  is  poised  in  proper  equilibrium.  The  upper 
screw  is  connected  by  the  wire,  TT,  with  an  electromagnet  at  the  back 
of  the  instrument;  a  corresponding  electromagnet,  partly  seen  at  M, 
is  connected  by  the  wire,  W',  with  the  screw  just  beneath  the  tip  of 
the  lever.  P  is  a  strong  clockwork  driven  by  the  cord,  T,  and  per- 
mitted to  run  intermittently  whenever  released  by  the  action  of  the 
electromagnets  at  M. 

The  clock  movement,  D,  regulated  by  the  pendulum,  F,  gives 
motion  to  the  recording  cylinders,  A,  A'.  The  cylinder,  A,  makes  1 
revolution  in  24  hours,  whereas  A'  revolves  at  the  much  slower  rate 
of  about  1  revolution  in  16  days. 

The  lever,  I,  together  with  the  platinum-pointed  screws  and  elec- 
trical connections,  W,  W,  are  all  mounted  upon  a  carrier,  R,  which  is 
moved  by  the  fine- threaded  screw,  S,  and  guided  by  the  columns  a,  a. 
The  coarse-threaded  screw,  S',  is  provided  with  the  double-pen  carrier, 
R',  the  screws,  S'  and  S,  being  geared  with  each  other  by  means  of 
suitable  wheels.  We  will  suppose  that  the  mechanisms  have  been 
properly  set  so  that  the  iron  float  is  normally  sustained  upon  the 
surface  of  the  mercury  and  the  lever,  I,  poised  in  equilibrium,  in  which 


BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        41 


FIG.  22.— Foreman's  barograph. 


42        BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

case  the  platinum  tip,  h,  will  stand  about  midway  between  the  plat- 
inum-pointed screws  above  and  below  it,  respectively.  The  pen  car- 
rier, R',  must  also  then  occupy  such  a  position  on  the  screw  that  the 
tracing  pens  will  indicate  the  true  barometric  pressure  upon  the 
rulings  on  the  record  sheet.  Appropriate  connections  with  an 
electric  battery  being  made,  the  action  of  the  mechanisms  will  be  as 
follows : 

Any  minute  change  in  the  level  of  the  mercury  will  alter  the  posi- 
tion of  the  iron  float,  in  consequence  of  which  the  platinum  tip  of  the 
lever,  Z,  will  move  into  contact  with  one  or  the  other  of  the  platinum- 
tipped  screws,  causing  a  current  of  electricity  to  be  directed  through 
the  electromagnet  connected  therewith.  The  action  of  either  electro- 
magnet releases  the  clockwork,  P.  In  doing  this,  however,  the  one 
magnet  shifts  the  lever,  L,  laterally  toward  the  back  of  the  instrument, 
while  this  lateral  movement  will  be  toward  the  front  if  brought  about 
by  the  action  of  the  other  magnet.  The  movement  of  the  clockwork 
causes  the  lever,  L,  to  advance  or  recede  so  that  a  pawl  upon  the  end, 
engaging  a  tooth  of  a  ratchet  wheel  upon  the  end  of  the  screw,  $', 
revolves  the  latter  a  fraction  of  a  turn.  This  fractional  turn  will  be 
in  one  direction  if  the  lever,  L,  is  drawn  backward  or  in  the  opposite 
direction  if  L  is  pushed  forward,  according  as  the  lever,  Z,  has  made 
contact  with  the  upper  or  under  screw.  This  movement  of  the  screw, 
$',  shifts  the  pen  carrier,  R' ,  and  the  pens  upon  the  record  sheet,  and, 
being  communicated  to  the  screw,  S,  causes  a  proportionate  change 
to  take  place  in  the  float-carrier,  R.  The  clockwork  automatically 
stops  after  one  such  cycle  of  actions.  If  after  these  movements  the 
lever,  /,  is  again  poised  in  equilibrium  no  further  action  ensues  until 
the  contact  of  h  with  one  or  the  other  of  the  screws  is  again  made, 
whereupon  the  cycle  of  actions  will  again  be  set  up  and,  if  necessary, 
repeated  in  quick  succession  until  the  equilibrium  of  the  poised  lever,  I, 
is  restored.  The  movement  of  the  pen  carrier,  Rf,  corresponding  to  a 
change  in  the  position  of  the  float  is  four  times  as  great  as  the  change 
in  the  height  of  the  mercurial  column.  A  change  of  1  inch  in  pressure, 
therefore,  is  represented  as  a  change  of  4  inches  on  the  sheet.  Each 
closure  of  the  circuit  producing  ^  revolution  of  the  screw,  Sf,  or  5V 
revolution  of  S  represents  a  change  in  the  height  of  the  mercurial 
column  of  0.001  inch,  which  is  the  nominal  sensitiveness  of  the  instru- 
ment. Owing,  however,  to  unavoidable  imperfections  in  screw 
threads  and  electric  contacts  and  to  the  capillary  action  of  the  mer- 
cury in  the  barometer  tube,  the  probable  error  of  the  instrument  is 
much  greater  than  this,  no  doubt  amounting  to  at  least  0.01  inch. 

By  selecting  proper  proportions  for  the  long  and  short  legs  of  a 
siphon  barometer,  the  effects  of  temperature  can  be  almost  perfectly 
eliminated.  This,  however,  appears  not  to  have  been  considered 
when  Foreman's  barograph  was  designed,  and  the  records  are  subject 


BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        43 

to  small  periodic  errors  due  to  temperature  changes.  For  the  further 
elucidation  of  the  automatic  compensation  of  siphon  barographs  for 
temperature  see  paragraph  80. 

Owing  to  the  presence  of  the  float  on  the  surface  of  the  mercury 
in  the  short  leg,  and  to  other  causes,  it  is  practically  impossible  to 
make  a  direct  measurement  of  the  actual  height  of  the  column  of 
mercury.  When,  therefore,  it  is  desired  to  set  the  recording  pens  or 
check  their  positions  in  relation  to  the  true  air  pressure,  it  is  neces- 
sary to  make  a  reading  of  the  standard  barometer.  In  the  barograph 
next  described  the  effect  of  temperature  is  inappreciable,  and  the 
actual  height  of  the  mercurial  column  may  be  directly  measured  at 
any  time,  thus  dispensing  with  the  extra  barometer  required  with 
Foreman's  barograph. 

71.  Marvin's  normal  barograph. — This  instrument  is  shown  in 
figures  23  and  24.  It  belongs  to  the  class  mentioned  above  under 
(4),  wherein  the  mercurial  column  is  directly  weighed  upon  a  balance. 

The  glass  tube  which,  with  the  top  portion  of  the  mercurial  column 
may  be  seen  at  B,  is  freely  suspended  by  the  hook,  h,  from  the  balance, 
A.  The  point  of  the  tube  dips  into  the  mercury  contained  in  the 
cistern,  (7,  which  is  suspended  by  a  gimbal  joint  from  the  columns,  d, 
d,  by  means  of  the  metal  tube,  B' ',  which  forms  a  sheath  and  protec- 
tion for  the  glass  barometer  tube  proper.  The  weight  of  the  barome- 
ter tube  on  the  short  arm  of  the  beam,  A,  is  balanced  by  the  rolling 
carriage,  W,  and  a  fixed  weight  (not  shown)  on  the  end  of  the  long 
arm  of  A.  Whenever  a  change  occurs  in  the  height  of  the  mercurial 
column,  the  weight  changes,  and  the  carriage,  W,  must  be  moved  to  a 
new  position  if  equilibrium  is  to  be  preserved.  In  order  to  make  the 
motions  of  the  carriage,  W,  automatic,  a  platinum-tipped  contact 
spring  is  attached  to  the  balance  beam  at  the  extreme  end,  r,  of  the 
long  arm.  The  slightest  displacement  of  the  beam  from  its  position 
of  equilibrium  causes  the  spring  to  move  into  contact  with  one  or  the 
other  of  two  platinum-pointed  screws,  shown  enlarged  in  figure  24  at 
m,  mf .  These  are  electrically  connected,  respectively,  with  the  mag- 
nets, M  and  M',  so  that  when  the  spring,  r,  makes  contact  with  m  or 
m',  an  electric  batter}r  being  in  proper  connection,  a  current  is  caused 
to  flow  through  the  corresponding  electromagnet,  the  action  of  which 
causes  the  pin,  N or  N',  to  engage  the  teeth  of  the  notched  wheel,  D, 
in  such  a  manner  as  to  revolve  it  tooth  by  tooth.  The  long  screw, 
S,  figure  23,  carries  the  wheel,  D,  fixed  at  its  end  so  as  to  be  revolved 
thereby.  The  threaded  carrier,  W,  fitted  to  the  screw,  8,  is  connected 
by  a  double  universal  linkage  to  the  rolling  carriage,  W.  The  electro- 
magnets thus  act  very  directly  through  the  wheel,  D,  and  the  screw,  8, 
to  automatically  move  the  carriage,  W,  into  such  positions  as  may 
be  required  to  maintain  the  equilibrium  of  the  balance:  that  is,  t<» 
prevent  the  contact  spring  on  the  beam  from  remaining  continuously 
in  contact  with  either  screw  m  or  m1 '. 


44        BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE 


FIG.  23.— Marvin's  normal  barograph. 


BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        45 

The  motor  mechanisms  act  in  such  a  manner  that,  whenever  the 
equilibrium  is  disturbed  and  the  electric  circuit  closed,  the  armature 
of  whichever  electromagnet  is  affected  makes  stroke  after  stroke, 
revolving  the  wheel,  D,  until  the  equilibrium  is  restored.  Generally 
one  or  two  strokes  only  are  necessary,  representing  a  change  in  the 


FIG.  24.— Electric-motor  mechanisms. 


carriage  corresponding  to  only  the  ten- thousandth  part  of  an  inch  of 
pressure,  as  explained  below. 

The  continuous  record  of  the  pressure,  as  indicated  by  the  succes- 
sive positions  of  the  rolling  carriage,  is  obtained  in  a  very  direct  and 
simple  manner.  A  suitable  spring,  adjustably  attached  to  the  threaded 


46        BAROMETERS   AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE. 

carrier,  W,  is  fitted  with  a  pen,  p,  figure  23,  and  traces  the  pressure 
curve  upon  a  large  cylinder,  not  shown  in  the  figure,  but  mounted 
with  its  axis  m  the  bearings  a,  af.  The  cylinder  is  revolved  regularly 
by  the  clock  movement,  C'. 

If  the  height  of  the  mercurial  column  changes  1  inch,  the  rolling 
carriage  and  recording  pen  will  move  5  inches,  thus  giving  a  sufficient 
magnification  to  render  estimations  of  the  pressure  to  the  one-thou- 
sandth part  of  an  inch  practicable. 

The  portion  of  the  long  arm  of  the  beam  over  which  the  rolling 
carriage  moves  is  provided  with  a  scale  of  20  subdivisions  to  the  inch 
which  represent  hundredths  of  an  inch  of  pressure.     Still  further 
subdivision  is  effected  by  reference  to  the  graduations  on  the  face  o: 
the  notched  wheel,  D.     These  represent  the  ten-thousandth  part  of  an 
inch  in  pressure.     Thus  a  mere  inspection  of  the  position  of  the  car- 
riage on  the  beam,  together  with  the  reading  on  the  notched  wheel 
gives  the  air  pressure  to  four  decimal  places.     Owing  to  the  frictiona 
resistances  and  other  influences  unavoidable  in  all  such  mechanisms 
the  fourth  figure  of  decimals  can  not  be  regarded  as  having  a  rea 
pressure  significance. 

The  readings  are  as  accurate,  probably,  as  the  best  eye  readings  oJ 
a  good  mercurial  barometer,  that-  is,  to  about  one-thousandth  part  oJ 
an  inch . 

The  record  is  not  appreciably  affected  by  changes  of  temperature 
that  affect  the  whole  instrument  uniformly. 

72.  Compensated   siphon  barograph,   Marvin  system. — This   instru- 
ment is  illustrated  in  figures  25  and  26  and  belongs  to  that  class  in 
which  the  record  is  made  mechanically  without  interposition  of  any 
clockwork  or.  electric  mechanisms  to  overcome  friction,  etc.     To  secure 
satisfactory  records  on  a  highly  magnified  scale  by  this  method  it  is 
indispensable  that  the  friction  involved  in  writing  the  magnifier 
record  be  removed  to  the  last  degree.     Experience  has  demonstratec 
that  this  has  been  accomplished  in  the  arrangement  described,  anc 
this  instrument  proves  to  be  exceedingly   accurate   and  far  more 
reliable  than  any  of  the  types  heretofore  employed.     The  clock  and 
electrical  mechanisms  required  in  the  older  instruments   act  in  a 
certain  sense   indirectly   and   are   the   cause   of  some   errors.     The 
weakening  of  batteries  or  failure  of  electric  mechanisms  from  time  to 
time  also  results  in  interruptions  in  tne  record  that  do  not  occur  in 
the  system  of  direct  mechanical  registration  now  to  be  described. 

73.  Compensated  siphon. — The  barometer  of  this  instrument  is  a 
special  form  of  siphon  clearly  shown  in  figure  25  and  with  dimensions 
marked  in  figure  26.     The  long  and  short  branches  consist  of  sim- 
ple, straight  tubes.     These  are  narrowed  down  at  the  lower  ends 
where  they  are  fitted  into  the  upturned  branches  of  the  bend,  or  U. 
The  tubes,  in  fact,  form  hollow  stoppers  carefully  fitted  and  ground 


BAROMETERS    AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        47 


T 


FIG.  25.— Compensated  siphon  barograph, 

Man-in  system. 
Glass  cover  case  removed. 


FIG  26.— Siphon  tube. 


48        BAROMETERS   AND  MEASUREMENT  .OF   ATMOSPHEEIC    PRESSURE. 

in.  The  tops  of  the  U  above  the  ground  joints  are  provided  with 
bells,  or  cups,  of  ample  size,  which  have  a  lip  formation  on  one  side. 
This  three-piece  construction  enables  the  barometer  to  be  filled  in  a 
most  satisfactory  manner,  but  more  especially  the  siphon  after  being 
once  filled  can  be  assembled  or  dismantled  and  transported  without 
loss  of  the  vacuum.  The  mercury  in  the  open  leg  of  the  siphon  in 
the  course  of  time  becomes  more  or  less  fouled  with  oxidation,  the 
accumulation  of  dust,  etc.  The  construction  described  permits  of 
removing  the  short  branch  of  the  siphon  at  any  time  with  very  little 
trouble.  The  tube  and  excess  of  mercury  can  then  be  thoroughly 
cleaned  and  replaced. 

74.  Filling  and  installing  the  siphon. — The  ordinary  siphon  tube 
made  in  one  piece  of  any  considerable  size  is  very  difficult  to  fill  and 
secure  a  good  vacuum,  and  it  can  not  then  be  easily  cleaned  or  trans- 
ported.    The  three-piece  construction  already  described  overcomes 
these  difficulties  and  requires  only  that  the  long  straight  branch  be 
carefully  filled.     This  may  be  done  by  almost  any  of  the  methods 
already  described  in  paragraphs  33  to  38,  but  the  air-pump  method 
is  undoubtedly  the  best. 

When  the  siphon  is  to  be  installed  it  will  be  well  to  prepare  the 
ground  joints  by  the  application  of  a  little  lubricant,  such  as  vaseline, 
tallow,  or,  if  available,  special  stopcock  lubricant,  very  sparingly 
rubbed  over  the  external  surfaces  of  the  tubes.  A  little  pure  mercury 
is  next  filtered  into  the  bend,  or  U-shaped  section.  Small  air  bubbles, 
if  any  appear,  should  be  excluded  by  tilting  the  tube  and  causing  the 
mercury  to  flow  about  in  a  manner  that  will  accomplish  this  result. 
When  the  mercury  covers  the  ground  surfaces  the  short  branch  of 
the  siphon  should  be  carefully  inserted  1  and  the  whole  secured  to  the 
instrument  in  the  manner  provided.  More  pure  mercury  is  now 
added  to  the  open  cup  until  it  is  filled  nearly  to  the  brim.  As  some 
mercury  is  likely  to  be  spilled  in  the  course  of  subsequent  operations, 
it  is  a  good  plan  to  have  a  clean  porcelain  or  glass  photographer's 
tray  close  underneath  the  plate  supporting  the  bend.  This  will  serve 
to  catch  any  mercury  that  may  escape. 

75.  The  long  branch  of  the  siphon,  completely  filled  with  clean 
mercury,  is  now  lifted,  and,  while  the  open  end  is  temporarily  closed 
firmly  with  the  finger  tip,  the  tube  is  carefully  inclined  in  a  manner 
that  will  permit  the  finger  and  point  to  be  dipped  below  the  free 
surface  of  mercury  in  the  cup.     Still  supporting  the  weight  of  the 
heavy  tube  so  that  the  submerged  end  does  not  bear  with  undue 
pressure  upon  the  parts  of  the  cup,  the  whole  is  carefully  and  slowly 
brought  into  a  vertical  position.     When  the  elevation  of  the  tube 
has  reached  the  point  at  which  the  mercury  begins  to  leave  the  top 

i  The  alignment  of  the  ground  joints  of  the  siphon  can  never  be  made  quite  perfect,  and  the  two  branches 
are  marked  with  a  side  that  when  faced  to  the  front  gives  the  best  results.  In  seating  the  tubes  they 
should  be  faced  in  this  position. 


BAROMETERS    AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        49 

of  the  tube,  an  assistant  should  be  ready  to  catch  in  a  suitable  vessel 
(a  dry,  clean,  drinking  glass  will  answer  very  well),  the  excess  of 
mercury  that  overflows  from  the  open  cup. 

The  heavy  tube  must  be  fully  supported  until  quite  vertical,  and 
the  end  only  then  inserted  into  the  ground  joint  and  rotated  a  little 
as  it  is  faced  to  the  front. 

76.  Certain  precautions  must  be  observed  throughout  the  opera- 
tions we  have  just  described.     (1)  The  tip  end  of  the  tube  must  not, 
under  any  circumstances,  be  lifted  out  of  the  mercury  after  the  finger 
is  removed.     (2)  After  the  flow  of  mercury  has  started  the  elevation 
of  the  tube  must  be  made  gradually;  otherwise  the  column  of  mer- 
cury will  tend  to  oscillate  or  surge  up  and  down  and  ma}'  uncover 
the  point  of  the  tube  in  the  cup.     (3)  Any  lowering  of  the  tube  causes 
the  mercury  to  recede  into  the  vacuum,  and  will  empty  the  cup  unless 
the  supply  is  kept  up  by  pouring  back  some  of  the  excess  that  has 
already  overflowed. 

77.  Having  finally  seated  the  long  branch,  some  of  the  excess  of 
mercury  must  be  restored  to  the  siphon  and  the  level  brought  up  to  the 
proper  point  in  the  open  leg.     At  the  completion  of  these  operations 
one  of  the  cups  of  the  bend  is  full  to  overflowing  with  mercury,  and 
the  other  is  nearly  or  quite  empty.     Some  of  the  mercury  in  the  full 
cup  can  easily  be  removed  by  splashing  it  out  into  a  cup  held  to 
receive  it,  using  a  piece  of  card  or  ivory  paper  folder  for  the  purpose. 
A  little  mercury  may  be  added  to  the  empty  cup. 

78.  To  clean  the  mercury. — When  the  glass  and  mercury  in  the 
open  leg  become  soiled  through  prolonged  use,  all  that  is  necessaiy 
ofter  removing  the  float  is  to  loosen  carefully  the  short  branch  of  the 
siphon  and  permit  the  excess  of  mercury  to  overflow  into  a  clean 
glass.     When  thus  emptied  the  open  branch  may  be  removed,  thor- 
oughly cleaned,  and  replaced.     Most  of  the  dirt  will  come  away  with 
the  glass  tube,  but  the  mercury  may  easily  be  filtered  and  replaced 
clean  and  bright. 

79.  To  dismantle  the  siphon. — If  it  is  desired  to  take  down  the 
siphon,  it  is  first  necessa^  to  remove  the  short  branch,  carefully  col- 
lecting the  excess  of  mercury,  and  then,  after  separating  the  ground 
joint  of  the  long  arm,  the  latter  is  slowly  inclined,  while  an  assistant 
steadily  pours  mercury  into  the  open  cups  to  replace  what  flows  into 
the  vacuum.     When  the  tube  is  entirely  filled,  the  finger  may  be 
slipped  over  the  open  end  while  submerged  in  the  mercury  and  the 
whole  tube  removed. 

80.  Temperature   compensation   of  siphon. — It   has    already   been 
mentioned  that  by  giving  the  siphon  barometer  proper  dimensions 
the  influence  of  temperature  can  be  eliminated  for  all  practical  pur- 
poses.    The   compensation   operates   so  that   changes   of  tempera- 
ture affecting  the  whole  instrument  uniformly  produce  no  sensible 

22937—12 4 


50        BAROMETERS   AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE 

change  in  the  level  of  the  mercury  in  the  short  or  open  branch  of  th 
siphon.  The  actual  difference  of  level  of  mercury  in  the  two  branche 
will,  of  course,  be  affected  by  temperature  in  the  usual  way,  but  no 
the  absolute  position  of  the  surface  in  the  open  leg.  Since  all  meas 
urements  are  made  only  on  this  surface  in  many  forms  of  mercuria 
barograph,  it  is  very  desirable  to  realize  in  the  design  of  such  instru 
ments  this  condition  of  automatic  compensation  for  temperature. 

The  physical  principle  utilized  for  this  purpose  is  found  in  th( 
different  rates  of  expansion  of  mercury  and  glass  or  whatever  mate 
rial  is  used  for  the  tube  or  envelope  for  the  mercury.  If  the  coefficien 
of  expansion  of  the  envelope  were  zero,  the  mercury  would  rise  slightly 
in  the  open  leg  with  rise  of  temperature,  and  vice  versa.  As  th< 
theory  of  this  temperature  compensation  is  not  stated  in  the  ordi 
nary  textbooks  of  physics  and  meteorology,  and,  in  fact,  does  no 
appear  to  be  widely  known,  it  seems  worth  while  to  present  it  her< 
briefly.  The  theory  was  developed  by  Prof.  G.  W.  Hough  1  in  1862 
and  later  by  Goulier:  2 

Let  m  =  Cubical  expansion  of  mercury  per  unit  temperature. 

Let  g  =  Cubical  expansion  of  glass  per  unit  temperature. 

Let  F0  =  Volume  of  mercury  in  instrument  at  temperature  tQ. 

Let  d  =  Diameter  of  tube  at  top  of  column  in  vacuum. 

Let  HQ  =  Height  of  column,  at  temperature  tQ. 

Let  H  =  Height  of  column,  at  temperature  t. 
dx,  d2  =  Diameter  of  the  two  branches  of  the  siphon  at  the  level  o 
the  top  of  the  column  in  the  open  branch. 

We  assume  that  the  pressure  remains  constant.  Therefore  th< 
barometric  column  for  a  change  of  temperature  must  change  it*. 
length  by  an  amount  represented  by  the  expression 

m(t-t0)H0; 
otherwise  its  hydrostatic  pressure  will  be  altered;  that  is, 


Neglecting  small  quantities  of  a  second  order  of  magnitude,  the  vol 
umetric  increase  in  the  barometric  column  will  be  the  expression 


which  is  the  change  necessary  to  preserve  hydrostatic  equilibrium 

1  Hough,  Prof.  G.  W.    Annals  of  the  Dudley  Observatory,  Albany,  N.  Y.,  Vol.  I,  1866,  p.  88. 

2  Goulier,  C.  M.    Comptes-rendus,  vol.  84,  1877,  p.  1315. 


BAROMETERS   AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        51 

Now,  the  apparent  change  in  the  volume  of  mercury  in  the  tube 
will  depend  upon  the  differential  expansion  of  mercury  and  glass, 
and  is  given  by  the  expression 

VQ(m-g)   (t-t0). 

When  this  increase  is  just  equal  to  that  necessary  to  preserve  hydro- 
static equilibrium,  all  the  expansion  will  seem  to  take  place  in  the 
vacuum  chamber  and  no  change  will  occur  in  the  level  of  the  mercury 
in  the  open  leg.  To  realize  this  condition  we  have 


V0(m-g)  (t-t0)  =  frd*m  (t-t0) 
m 


4       rn  —  g 
The  expression  — j— 

is  the  volume  of  the  barometric  column,  supposing  the  diameter  to  be 
the  same  throughout  as  at  the  top. 

The  cubical  coefficient  of  expansion  of  mercury,  m,  is  a  very  definite 
quantity  and  for  barometric  work  may  be  taken  to  be  0.0001010 
per  degree  Fahrenheit.  The  expansion  of  glass  is  much  smaller  and 
varies  considerably,  ranging,  according  to  Regnault's  measurements, 
from  0.0000145  for  common  white  tubing  to  0.0000118  for  the  hard 
French  and  crystal  tubes.  That  is  to  say,  the  whole  volume  of  mer- 
cury in  a  siphon  to  be  compensated  must  be  about 


if  made  of  common  tube, 

or  F0=  1.132 

if  made  of  French  crystal. 

Xo  great  exactness  is  necessary  in  the  volume  of  F0.     It  will  suffice 
[to  assume  H0=  the  mean  barometric  pressure  at  the  place  of  observa- 
tion, and  the  total  volume  of  mercury  should  be  about  17  per  cent 
[more  than  requisite  to  fill  a  column  of  height  H0  and  diameter  d. 
If  the  bend  of  the  siphon  is  of  wide  bore,  the  open  leg  and  bend 
ust  be  very  short  (for  example:  30x0.17  =  5.1  inches);  otherwise 
F0  will  be  too  large.     For  this  reason,  as  well  as  for  convenience  of 
:onstruction,  the  bend  is  best  made  of  smaller  diameter  than  the  main 
ube,  as  shown  in  the  illustration. 

81    The  theory  given  above  takes  account  only  of  the  influence  of 
uperature  on  the  mercury  and  glass  tube.     The  effects  that  result 
changes  in  the  mechanisms  described  later  for  transmitting  and 
inscribing  the  record,  and  for  holding  the  glass  barometer  tube  itself, 


52        BAROMETERS    AND    MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

all  require  consideration,  but  fortunately  these  are  in  the  main  sc 
small,  especially  when  considered  in  relation  to  the  highly  magnified 
scale  on  which  the  record  is  inscribed,  that  they  may  be  neglected.     I 
any  case  they  can  be  incorporated  with  the  mercury  effect  so  that  b 
adding  or  removing  small  amounts  a  certain  total  volume,    F0,  of 
mercury  at  temperature  t0  may  be  employed,  and  thus  all  uniform 
effects  of  temperature  on  the  whole  apparatus  will  be  automatically 
compensated. 

If  .the  siphon  is  not  compensated,  then  the  volume  of  mercury  at 
temperature  10  =  Vl}  which  in  general  will  be  greater  than  F0,  but 
may  be  less,  and  a  small  correction  will  be  required,  the  amount  of 
which  will  be  simply  the  apparent  expansion  of  the  excess  of  mercury 
occupying  the  bend  and  short  leg  of  the  siphon.  This  expansion 
may  be  imagined  simply  to  lift  the  whole  column  of  mercury  a  small 
amount,  All. 

The  volumetric  expansion  will  be 

(Fj-Fo)   (m-g)   (t-Q, 
and  the  rise  of  mercury,  All,  is  given  by  the  expression 

^(dl  +  dl)  Ah=(V,-V»)  (m-g)(t-tQ). 

In  general,  d^  and  d2  will  be  made  sensibly  equal,  and,  in  fact,   =d. 
Hence: 

Jft  =  9(F      F)(m-.9)(*-O 
1        °  nd2 

Let  y=  the  amount  by  which  the  mercury  in  the  open  leg  of  tli 
siphon  stands  higher  than  required  for  compensation.  Then,  sine 
an  equal  excess  of  mercury  occupies  the  opposite  branch  of  the  I 
we  have: 


Hence, 

A~h=y  (m-g)    (t-Q, 
or,  for  ordinary  glass: 

Ah=. 0000865  ?/  (t-t0). 

That  is,  the  temperature  correction  required  is  simply  the  apparen 
expansion  of  the  short  column  y  of  excess  of  mercury.  Now,  th 
temperature  variations  affecting  a  barograph  throughout  the  perio 
of  a  single  record  sheet,  or  rather  during  intervals  when  the  recor 
may  be  checked  by  eye  observations  of  a  standard  barometer,  wi 
rarely  exceed  10°  or  20°  F.  If  y=l  inch,  for  example,  then  All  fo 
20°  =.00173,  a  quantity  which,  in  general,  may  be  neglected. 


BAROMETERS    AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        53 

82.  Magnifying  and  recording  mechanisms. — In  the  barograph  illus- 
trated, the  barometric  changes  are  magnified  five  times  and  recorded 
on  a  vertical  drum  adapted  to  embrace  a  change  of  2  inches  of  pres- 
sure and  revolving  once  in  three  days,  moving  at  the  rate  of  nearly 
a  quarter  of  an  inch  per  hour.  A  long  experience  with  a  variety  of 
scales  indicates  that  records  on  time  and  pressure  scales  of  about  the 
above  proportions  give,  on  the  whole,  the  most  satisfactory  and 
graphic  picture  of  ordinary  barometric  oscillations.  Even  the  sud- 
den changes  that  sometimes  occur  with'  thunderstorms  are  very  well 
brought  out;  but  for  the  most  detailed  effects  of  this  character  a 
more  rapid  time  scale  is  necessary.  The  magnification  is  sufficiently 
great  to  show  clearly  the  small  fluctuations  of  from  a  few  thousandths 
to  some  hundredths  of  an  inch  that  sometimes  occur  for  hours  at  a 
time. 

In  the  siphon  form  of  barometer  the  change  of  level  of  the  mercury 
in  either  leg  is  only  half  of  the  whole  change,  assuming  both  legs  to 
have  the  same  diameter,  and,  since  we  measure  effects  in  the  open  leg 
only  and  desire  a  fivefold  magnification,  it  follows  that  an  actual  ten- 
fold magnification  of  the  movements  of  the  float  is  necessary.  This 
is  accomplished  by  a  large  and  a  small  wheel  operating  on  the  prin- 
ciple of  the  wheel  and  axle,  as  may  be  clearly  seen  in  figures  25  and  27. 
This  construction  provides  a  perfectly  balanced  system  which  is  itself 
neutral  in  all  positions  and,  at  the  same  time,  admits  of  a  wide  range 
of  movement,  results  impossible  to  secure  with  lever  systems  com- 
monly employed  in  cases  of  this  kind. 

In  order  to  secure  strength  of  construction  and  at  the  same  time 
reduce  friction  to  a  minimum,  the  multiplying  wheel  and  axle  are 
mounted  on  carefully  designed  and  constructed  ball  bearings,  each 
cell  containing  only  six  balls,  each  one-sixteenth  inch  in  diameter. 
The  ends  of  the  axis  entering  the  ball  cups  are  70°  cones. 

A  conical  steel  float,  with  the  base  somewhat  hollowed  out  so  as 
to  conform  fairly  well  with  the  shape  of  the  surface  of  mercury  rests 
lightly  upon  the  top  of  the  column  and  is  suspended  from  the  small 
drum  of  the  wheel-and-axle  system  by  means  of  a  narrow  platinum 
ribbon  about  0.001  inch  thick.  The  pen  carrier  is  suspended  by  a 
very  fine  copper  wire  running  in  a  groove  in  the  rim  of  the  large 
wheel,  the  diameter  of  which  is  approximately  5  inches,  while  that 
of  the  drum  is  one-tenth  as  great.  The  exact  ratio  of  these  wheels  is 
made  so  as  to  realize  a  fivefold  magnification  of  pressure  changes; 
due  account  being  taken  of  any  slight  dfferences  in  the  diameters 
of  the  open  and  closed  chambers  of  the  barometric  column. 

To  realize  a  condition  of  minimum  friction  great  attention  is  neces- 
sary in  the  design  and  arrangement  of  the  pen  carrier.  First,  the 
weight  must  be  the  least  practicable,  since  the  mass  of  the  float  must 
be  somewhat  in  excess  of  10  times  that  of  the  pen  carrier,  and  any 


54        BAROMETERS    AND    MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

unnecessary  weight  in  these  parts  introduces  avoidable  pressure  and 
friction  on  the  axle.  Second,  while  the  pen  carrier  is  guided  and 
constrained  to  move  without  sensible  looseness  in  a  definite  vertical 
line  by  sliding  along  a  fine,  stretched  wire,  nevertheless  the  whole 
arrangement  is  so  poised  and  balanced  that  if  not  disturbed  by  exte- 
rior influences  the  carrier  will  rise  and  fall  in  exactly  the  same  ver- 
tical line,  as  nearly  as  may  be,  even  when  the  wire  is  removed.  This 
adjustment  serves  to  eliminate  any  sliding  friction  experienced  by 
the  pen  carrier  not  absolutely  essential  to  constraing  the  pen  to 
the  desired  vertical  line.  Finally,  the  contact  pressure  of  the  pen  on 
the  record  sheet  is  no  greater  than  essential,  and  results  from  a  small 
residual  gravitational  tendency  of  the  carrier  to  rotate  the  pen  point 
against  the  record  sheet  with  a  very  gentle  pressure.  The  siphon 
tubes  are  about  1J  inches  in  diameter  and  the  float  is  only  slightly 
smaller.  This  gives  a  moving  force  capable  of  overcoming  the  una- 
voidable friction  in  a  highly  satisfactory  manner,  and  the  absence  of 
any  complicated  mechanisms  renders  false  and  interrupted  records 
almost  an  impossibility. 

83.  Time  checks  on  record  sheet. — As  thus  far  described  the  baro- 
graph is  complete,  and  with  the  aid  of  the  driving  clock  and  drum, 
which  require  no  further  description,  gives  highly  accurate  and 
continuous  records.  The  detailed  analysis  of  barometric  records 
generally  requires  hourly  readings.  When  record  sheets  are  employed 
with  ruled  scales  for  pressure  and  time,  there  is  always  a  difficulty  in 
setting  the  record  so  that  the  ruled  hour  lines  on  the  sheet  indicate 
the  true  time.  A  similar  difficulty  arises  in  setting  the  pen  to  the 
correct  point  on  the  pressure  scale.  This,  however,  is  of  slight  con- 
sequence if  sheets  are  properly  printed  and  cut  with  uniform  margins 
and  carefully  placed  on  the  cylinder. 

To  secure  easily  an  equally  satisfactory  result  with  the  time  record, 
the  driving  clock  is  provided  with  a  dial  and  hands  in  the  usual  fashion. 
These  moving  continuously  day  after  day  enable  the  clock  to  be  reg- 
ulated to  keep  correct  time,  a  result  very  hard  to  secure  when  the 
rating  is  done  on  record  sheets  that  are  frequently  changed.  More 
especially,  however,  the  barograph  is  equipped  with  a  special  time- 
marking  device  which  automatically  operates  once  each  hour  at  the 
instant  the  minute  hand  of  the  clock  reacher  XII,  or  the  zero  point 
of  the  hour.  Nearly  all  the  time  lines  are  omitted  from  the  printed 
rulings  of  the  record  sheet,  and  the  marker  operates  so  as  to  lift  the 
float  a  few  hundredths  of  an  inch  and  immediately  release  it.  This 
causes  the  recording  pen  to  oscillate  a  few  times  up  and  down  and 
to  inscribe  a  short  transverse  line  across  the  pressure  record.  These 
transverse  strokes  are,  in  fact,  the  hour  lines  for  the  entire  record 
and  are  inscribed  with  all  the  accuracy  required. 


BAROMETERS   AND  MEASUREMENT   OF   ATMOSPHERIC    PRESSURE.        55 

84.  There  is  a  further  advantage  from  the  action  of  the  time 
marker.  The  float  is  raised  from  the  mercury  momentarily  and 
subsequently  oscillates  slightly.  This  causes  a  general  renewal  of 
the  forces  of  buoyancy  and  capillarity  which  determine  the  exact 
position  of  the  float,  and  any  failure  of  the  pen  at  the  end  of  the  oscil- 
lations to  return  exactly  to  its  original  position  is  an  index  of  the 
magnitude  of  errors  that  arise  partly  from  friction  and  partly  from 
variation  in  the  capillary  and  buoyant  forces. 

Discontinuities  of  several  thousandths  of  an  inch,  due  to  these 
causes,  are  sometimes  found  in  the  records,  "rendering  apparent  the 
existence  of  small  errors  which  would  otherwise  be  only  conjectured. 


FIG.  27.— Time  marker.  •, 

85.  The  time  marker  is  shown  in  figure  27,  and  consists  of  an  elec- 
tromagnet, the  circuit  of  which  is  momentarily  closed  by  a  spring 
contact  operated  by  the  minute  hand  of  the  clock  as  it  passes  the 
XII  point  of  the  dial. 

The  armature  of  the  magnet  is  L-shaped,  as  seen  in  the  picture. 
A  long,  light,  horizontal  rod  is  pivoted  at  the  depressed  end  of  the 
L-formed  armature,  and  is  also  partly  lifted  by  the  pull  of  a  spring 
carried  wholly  on  the  armature.  The  outer  end  of  the  long  arm  is 
tipped  with  a  bit  of  soft  rubber,  and  is  further  loaded  with  a  small 
counterweight,  which  rests  lightly  on  a  small  post  or  stop  provided 
for  that  purpose. 


56        BAROMETERS    AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

86.  The  action  of  the  marker  is  as  follows:  When  the  armature  is 
suddenly  pulled  down  upon  the  magnet  the  rubber-tipped  rod  is 
thrust  forward  against  the  rim  of  the  large  wheel.  The  inertia  of 
the  counterweight  suffices  to  overcome  for  an  instant,  but  only  for 
an  instant,  the  pull  of  the  spring  previously  mentioned.  In '  this 
instant,  however,  the  rubber-tipped  end  of  the  rod  has  engaged  the 
rim  of  the  wheel  and  the  slower-acting  pull  of  the  spring  then  lifts 
the  rod  and  thus  turns  the  wheel  a  small  distance  (one  to  two  tenths 
of  an  inch) .  As  soon  as  the  armature  is  released  by  the  breaking  of 
the  contact  in  the  clock,  the  wheel  and  float  are  released  and  oscil- 
late freely  for  a  moment,  producing  the  results  already  fully 
explained. 


FIG.  28.— Richard's  aneroid  barograph. 

87.  The  recording  drum  makes  a  complete  rotation  in  74  hours, 
i.  e.,  3  days  and  2  hours.     Sheets  ordinarily  are  changed  at  any  time 
between  11   a.  m.  and  12  noon,  preferably  shortly  after  11   a.  m. 
The  new  record  is  therefore  fully  started  before  noon,  and  a  check 
reading  of  the  standard  barometer  is  made  as  nearly  as  possible  at 
noon.     This  furnishes  a  check  observation  for  determining  the  start- 
ing error  of  the  barograph.     Further  checks  may  be  obtained  sub- 
sequently from  the  regular  observations  at  8  a.  m.  and  8  p.  m. 

88.  Aneroid  barographs.— Extremely   simple   and   portable   baro- 
graphs are  constructed  upon  the  aneroid  principle,  of  which  that  of 
Richard,  being  widely  used,  is  fully  described.     (See  fig.  28.) 

It   consists   of  a   cylinder,  A,   on   which  the   recording  paper  is 
wound,  revolving  once  a  week  by  means  of  a  clockwork  contained 


BAROMETERS   AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        57 

inside.  A  series  of  corrugated  metallic  shells,  B,  eight  in  number,  joined 
one  above  the  other  and  exhausted  of  air,  forms  an  aneroid  system 
eight  times  as  sensitive  as  a  single  chamber.  The  movement  of  the 
shells  is  still  further  greatly  magnified  and  is  transmitted  to  the  record- 
ing pen,  Cj  by  a  series  of  connecting  levers.  The  pen  may  be  released 
from  contact  with  the  paper  by  pushing  the  lever,  D,  to  the  right. 

The  corrugated  shells  are  the  same  as  used  in  ordinary  aneroids,  as 
described  in  paragraph  46,  the  steel  springs  for  distending  the  shells 
being  placed  inside.  The  shells  are  made  into  a  vertical  column  by 
screwing  the  one  on  the  other.  The  lower  base  of  the  column  being 
1i\cd,  the  upper  end  rises  and  falls  with  every  variation  in  the  atmos- 
pheric pressure,  by  a  quantity  which  is  the  sum  of  the  displacements 
of  the  elementary  shells. 

The  compensation  for  temperature  is  accomplished  by  leaving  a 
sufficient  quantity  of  air  in  one  of  the  shells,  ascertained  by  experi- 
ment when  the  instrument  is  made,  so  that  with  a  rise  of  temperature 
the  tendency  of  the  barometer  to  register  too  low  on  account  of  the 
weakening  of  the  springs,  and  the  expansion  of  the  levers  and  other 
parts  is  counteracted  by  the  increased  pressure  of  the  air  in  the  shell. 
However,  the  instrument  should  be  kept  at  a  uniform  temperature 
as  far  as  possible. 

V. — GENERAL    INSTRUCTIONS. 
(A)   FOR   CARE    AND    USE    OP    BAROMETERS. 

89.  Exposure  of  barometers. — The  two  important  considerations  in 
selecting  a  proper  location  for  a  barometer  are  (1)  that  the  cistern 
and  top  of  the  mercurial  column  may  be  in  a  good  light  and  (2)  that 
the  temperature  may  be  as  constant  as  possible.  The  best  conditions 
for  light  are  obtained  when  the  barometer  can  be  placed  between  the 
observer  and  a  window,  preferably  a  north  one,  covered  either  with 
tissue  paper  or  fitted  with  ground  glass.  Very  nearly  as  good  results 
are  obtained  by  a  light  from  one  side  reflected  from  clean  white 
paper  or  white  glass  immediately  back  of  the  barometer.  The  top 
of  the  column  should  be  about  the  height  of  the  observer's  eye.  The 
barometer  should  not  be  exposed  either  to  the  direct  rays  of  the  sun 
or  to  the  air  currents  that  are  always  found  in  the  vicinity  of  cracks 
and  crevices  in  windows. 

In  establishing  stations,  officials  will  use  special  care  in  selecting 
the  exposure  of  the  barometers  and  satisfy  the  conditions  stated  above 
as  nearly  as  possible.  In  general,  it  will  be  necessary  to  avoid  expo- 
sures near  windows,  as  proper  temperature  conditions  can  not  be 
found  in  such  locations. 

As  houses,  no  matter  how  tightly  built,  always  permit  the  free  flow 
of  air  in  and  out  through  crevices,  ventilators,  chimneys,  etc.,  it 


58        BAROMETERS    AND   MEASUREMENT    OP    ATMOSPHERIC    PRESSURE. 

results  that  the  air  pressure  within  is  exactly  the  same  as  without, 
except  possibly  for  very  slight  differences  of  very  short  duration.  If 
such  were  not  the  case  it  would  be  necessary  to  expose  barometers 
out  of  doors  to  obtain  the  real  air  pressure. 

89a.  Pumping  of  barometers. — Notwithstanding  what  has  just 
been  said  about  the  pressure  indoors  and  out  being  the  same  during 
very  windy ,  gusty  weather  barometers  within  doors  are  often  sub- 
jected to  very  rapid  and  irregular  oscillations  of  pressure,  caused  by 
gusts  of  wind  blowing  into  doorways,  windows,  or  chimneys,  and 
momentarily  increasing  the  pressure,  or  by  blowing  across  chimney 
tops  and  otherwise,  so  as  to  produce  a  sort  of  suction  that  momen- 
tarily diminishes  the  general  pressure.  In  consequence  of  these 
effects  the  mercurial  column  of  a  barometer  may  be  observed  on 
such  occasions  to  rise  and  fall  irregularly  within  narrow  limits,  the 
motion  in  many  cases  being  little  more  than  changes  in  the  curvature 
.of  the  meniscus.  This  action  is  called  the  "pumping"  of  barometers, 
and,, of  course,  interferes  with  accurate  pressure  observations. 

The  term  "pumping"  is  also  applied  to  much  more  violent  oscil- 
lations of  the  mercurial  column,  such,  for  instance,  as  will  occur 
when  an  ordinary  barometer  is  exposed  on  a  vessel  at  sea,  or  when 
carried  in  an  upright  position  in  the  hand.  In  barometers  for  use  on 
shipboard  this  action  is  prevented  by  making  the  lower  portion  of 
the  glass  tube  of  very  fine  bore,  so  that  the  movement  of  the  mer- 
cury is  necessarily  too  slow  to  follow  sudden  and  irregular  oscillations 
of  pressure. 

90.  Verticality  of  barometers. — For  accurate  results  it  is  necessary 
that  barometers  should  be  exactly  vertical  when  the  adjustments  for 
reading  are  made.     For  this  purpose  the  better  forms  of  barometers 
are  arranged  to  be  suspended  from  rings  at  the  top,  so  that  the  instru- 
ment itself  acts  as  a  plumb  line  and  takes  a  vertical  position  with 
sufficient  accuracy.     It  is  desirable,  however,  for  convenience  in  set- 
ting the  barometer,  as  well  as  to  insure  the  permanent  verticality 
of  the  instrument,  to  steady  it  in  supports  which  are  first  adjusted, 
once  for  all,  so  that  the  barometer  is  accurately  vertical,  as  determined 
by  a  plumb  line  applied  alternately  at  the  front  and  one  side. 

91.  Improved  barometer  box. — The  standard  pattern  of  barometer 
box  now  in  use  by  the  Weather  Bureau  is  shown  in  figure  29. 

This  box  must  be  securely  attached  to  the  wall  in  a  location  afford- 
ing good  light  and  not  subject  to  sudden  changes  of  temperature. 
In  many  cases  it  will  first  be  necessary  to  fasten  to  the  wall  hard- 
wood strips,  to  which  the  top  and  bottom  of  the  barometer  box  can 
be  secured  by  screws  passing  through  metal  plates  provided  for  this 
purpose.  Place  the  screw  for  holding  the  top  of  the  box  in  the  { 
center  of  the  top  strip  and  suspend  the  box  thereon.  When  the 


BAROMETERS    AND    MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        59 


FIG  29.— Improved  barometer  box. 


60        BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 


box  is  set  about  vertical  (as  determined  by  a  plumb  line)  secure  it 
firmly  at  the  bottom  by  a  screw  passing  through  the  metal  plate  into 
the  wooden  strip. 

The  hooks  in  the  top  of  the  barometer  box  will,  upon  examination, 
be  found  to  be  adjustable  in  their  positions.  Place  them  in  their 
central  positions  and  hang  thereon  the  ''station"  and  "extra"  barom- 
eters. Next,  find  the  positions  at  the  bottom  for  attaching  the  ring- 
shaped  guides,  one  of  which  is  shown  in  figure  30.  These  rings 
should  be  slipped  over  the  lower  end  of  the  barometer,  and  placed 
about  the  mid  height  of  the  cylindrical  part  of  the  cistern.  Mark 
the  screw  holes  and  fasten  the  rings  to  the  back  of  the  box.  The 
barometer  when  at  rest  should  hang  freely  within  this  ring,  and  may 

be  adjusted  to  do  so  by  the  hook  at  the 
top.  When  the  rings  are  fitted  with  cen- 
tering screws  these  should  be  careful!}' 
screwed  up  until  the  barometer  cistern  is 
very  gently  clamped  and  held  steadily, 
but  not  in  the  least  deflected  from  its 
vertical  position  when  free. 

Barometers  that  swing  slightly  free 
within  the  ring  support  should,  in  setting, 
be  steadied  against  one  side  of  the  ring, 
as  shown  in  figure  30.  The  following 
caution  must,  however,  be  observed. 

92.  Caution  against  error. — In  adjusting 
the  barometer  for  reading  it  is  very  im- 
portant that  it  be  steadied  against  the 
ring  in  the  proper  manner,  otherwise  an 
appreciable  error  is  introduced,  because  of 
imperfect  verticality. 

If  7,  figure  30,  is  the  position  of  the 
ivory  point,  then  steady  the  cistern  against 
the  ring  at  A  or  at  B,  but  never  at  C  or  I),  or  at  other  points.  A 
and  B  should  always  be  in  a  line  exactly  at  right  angles  to  a  line  through 
the  ivory  point  and  the  center  of  the  cistern. 

A  little  thought  will  show  the  necessity  for  this.  If,  for  illustra- 
tion, we  imagine  the  ivory  point  at  one  side  and  just  in  contact  with 
the  mercurial  surface  and  the  barometer  vertical,  it  is  very  clear  that 
if  we  swing  the  barometer  out  of  vertical  a  little  either  to  one  side 
or  the  other,  the  ivory  point  will  either  dip  into  the  mercury  a  little 
or  rise  above  the  surface,  whereas,  if  the  barometer  be  deflected  back- 
ward or  forward,  there  will  be  little  or  no  perceptible  change  in  the 
level  of  the  mercury  at  the  ivory  point. 


FIG.  30.— Barometer  cistern  and 
ring  support. 


BAROMETERS   AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        61 


93.  Old-style  barometer  boxes. — The  simplest  form  of  box  used  by 
this  bureau  is  shown  in  an  improved  form  in  figure  31.     The  top  end 
of  the  box  is  made  of  thick  material,  which  is  recessed  to  receive  the 
top  portion  of  the  barometer  when  suspended  upon  the  long  hook,  77, 
screwed  into  the  top.     For  reading,  the  barometer  is  drawn  out  upon 
the  hook,  as  shown  in  the  figure,  and  returned  to  the  box  after  the 
observation.     The  lid  of  the  box  is  fitted  to  close 

under  the  hook  so  that  the  whole  barometer  is 
thoroughly  encased  and  well  protected.  Except  that 
the  barometer  must  be  shifted  out  of  and  into  the 
box  every  time  readings  are  made,  and  the  further 
disadvantage  that  the  cistern  necessarily  swings  free, 
this  style  of  box  answers  the  purpose  in  a  very  satis- 
factory manner. 

94.  The  barometer  should  be' carefully  lifted  along 
the  hook  and  not  made  to  slide  roughly  or  permitted 
to  knock  against  the  guides  in  the  box. 

95.  When  boxes  such  as  described  in  the  preced- 
ing paragraphs  are  not  furnished  the  barometer  may 
be  suspended  from  almost  any  suitable  hook  securely 
fastened  to  the  wall  in  such  location  as  will  satisfy,  as 
far  as  possible,  the  conditions  of  paragraph  89. 

96.  Marine  barometer  box. — This  has  already  been 
described  in  previous  paragraphs  26  to  31.     We  need 
only  add  that  in  attaching  this  box  to  the  wall  the 
height  must  be  regulated  so  that  the  scale  comes  at 
the  level  of  the  eye  when   the  hinged   bracket  is 
lowered  with  the  barometer  in  the  reading  position. 
The  barometer  will  be  several  inches  too  high  to 
read  conveniently  when  folded  into  the  box. 

97.  How  to  set  and  observe  the  barometer. — Having 
in  mind  the  various  sources  of  error  affecting  barome- 
ters, and  other  peculiarities  of  the  instrument,  we  may 
next  consider  how  best  to  secure  accurate  readings. 
The  presence  of  the  observer's  body  near  the  barome- 
ter ends  to  increase  its  temperature.     The  scale  and 
outer  parts  are  affected  first,  then  the  thermometer, 
and  much  more  slowly  the  mercurial  column.     Gen- 
erally, however,  this  effect  is  slight,  as  only  a  few 

minutes  are  required  in  making  a  reading.  It  is  best  to  read  the 
attached  thermometer  first.  Next,  if  the  barometer  is  freely  sus- 
pended, jostle  the  cistern  a  little,  so  that  the  mercurial  surfaces  may 
be  detached  if  they  tend  to  cling  to  the  glass  walls.  To  "set"  the 
cistern  of  the  barometer,  the  level  of  the  mercury  should  be  lowered 
a  little  by  turning  the  milled  head,  0,  figure  3,  and  raised  again  until 


Fro.  31.— Barometer 
box. 


62        BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE. 

it  is  just  in  contact  with  the  ivory  point.  To  make  this  adjustment 
of  the  mercury  to  the  ivory  point  accurately  requires  care  and  sharp 
scrutiny. 

Adjustment  of  cistern,  first  method. — One  way  is  to  sight  between  the 
point  and  the  mercury  and  watch  for  the  slightest  thread  of  light  that 
can  be  detected.  The  screw  should  be  turned  very  carefully  until 
this  thread  of  light  just  disappears.  This  method  is  believed  to  be 
best  and  is  uniformly  practiced  at  the  central  office.  It  is  equally 
applicable  to  new  barometers  with  bright  mercurial  surfaces  and  to 
older  ones  the  mercury  of  which  is  more  or  less  oxidized.  The  light 
should  be  strong  from  behind  the  barometer  and  the  front  of  the 
cistern  should  be  in  shadow. 

Second  method. — The  adjustment  of  the  mercury  to  the  ivory  point 
may  also  be  determined  by  watching  the  formation  and  disappearance 
of  the  small,  dimplelike  depression  made  in  the  mercury  when  the 
ivory  point  is  pressed  into  the  mercury  a  little  and  again  withdrawn. 
When  the  dimple  just  disappears  the  surface  may  be  supposed  in  con- 
tact with  the  ivory  point.  The  mercury  often  clings  to  the  ivory 
point,  especially  when  the  ivory  is  newly  cut.  This  method,  however, 
is  not  so  reliable  and  accurate,  and,  in  general,  can  be  followed  only 
with  clean  mercury.  Moreover,  it  is  not  good  practice  to  lower  the 
mercury  any  slight  amount  after  it  is  once  raised  to  the  ivory  point. 
The  effect  of  this  generally  is  to  change  simply  the  convexity  of  the 
meniscus  at  the  top  of  the  column,  and  this  gives  rise  to  a  new  and 
unknown  correction  for  capillarity.  The  most  uniform  results  are 
obtained  by  gradually  raising  the  mercury  until  precise  contact  is 
secured.  If  it  is  imagined  the  mercury  has  been  raised  too  much, 
lower  it  until  entirely  free  from  the  point  and  adjust  again. 

Third  method. — Another  method  that  is  often  given  is  to  watch 
closely  until  the  reflected  image  of  the  ivory  point  coincides  with  the 
point  itself.  This  also  requires  clean,  bright  mercury  and  is  there- 
fore not  a  general  method.  Great  precision  in  the  adjustment  of  the 
contact  of  the  ivory  point  with  the  mercury  may  be  attained  with  a 
little  care  and  practice,  and  observers  may  scarcely  be  conscious  of 
precisely  the  manner  of  making  the  adjustment. 

98.  Adjustment  of  the  vernier.— The  level  of  the  mercury  being 
adjusted  to  the  ivory  point,  the  vernier  must  next  be  brought  to  the 
top  of  the  column.  Greater  uniformity  and  accuracy  are  insured  if 
the  fingers  be  now  tapped  smartly  against  the  side  of  the  metal 
barometer  tube.  This  aids  the  mercury  in  detaching  itself  from  the 
glass  and  forming  into  a  normal  meniscus.  The  proper  setting  of  the 
vernier  is  made  when  the  light  is  just  cut  off  from  across  the  extreme 
summit  of  the  meniscus.  The  figures  on  page  13  indicate  how  the 
vernier  should  be  set  to  the  mercurial  meniscus.  The  lower  edge  of 
the  vernier  must  be  brought  just  to  the  level  of  the  extreme  summit  of 


BAROMETERS    AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        63 

the  meniscus.     The  eye  must  be  held  so  that  both  front  and  back  edges 
of  the  vernier  are  in  the  line  of  vision. 

99.  It  is  needless  to  say  that  throughout  the  setting  of  the  barome- 
ter, as  described  above,  the  column  must  be  maintained  vertical, 
either  by  means  of  the  fixed  supports  or  by  the  skillful  handling  of  the 
freely  suspended  barometer,  so  that  at  the  critical  moments  when 
contacts  are  judged  to  be  made  the  instrument  is  truly  vertical. 

100.  After  the  cistern  and  vernier  are  adjusted  in  the  manner 
described  above  it  remains  only  to  read  the  scale  and  vernier  in 
accordance  with  the  instructions  in  paragraphs  13,  14,  and  15. 

101.  This  reading  may  next  be  corrected  for  temperature  by  apply- 
ing the  proper  correction  taken  from  Table  I,  corresponding  to  the 
temperature  shown  by  the  attached  thermometer,  and  further  cor- 
rected, if  necessary,  by  the  addition  or  subtraction,  as  the  case  may  be, 
of  all  other  corrections  known  for  the  instrument,  such  as  correction 
for  capillarity,  instrumental  error,  imperfect  vacuum,  gravity,  etc. 

102.  Reduction  to  standard  gravity. — The  following  will  elucidate 
the  nature  of  the  gravity  correction  as  applied  to  barometric  observa- 
tions— an  important  matter  that  is  often  but  indifferently  considered 
in  the  ordinary  textbooks  of  meteorology : 

By  the  well-known  principle  of  hydrostatics  on  which  the  action 
of  the  mercurial  barometer  is  based  the  pressure  of  the  atmosphere 
is  equal  to  the  pressure  of  the  column  of  mercury  that  it  will  support. 
But  this  latter  pressure  is  only  another  name  for  the  weight  of  the 
mercury,  and  for  columns  of  equal  section  the  weight  varies  both 
with  the  height  of  the  column  and  with  the  force  of  gravity. 

The  force  of  gravity  varies  with  latitude  and  altitude;  therefore  the 
height  of  the  barometer,  even  when  corrected  for  temperature  and 
instrumental  error,  does  not  give  us  a  true  measure  of  the  atmospheric 
pressure  unless  we  first  eliminate  the  small  variations  that  are  due  to 
gravity;  that  is,  observations  taken  over  a  widely  extended  region  to 
be  strictly  comparable  must  be  reduced  to  a  standard  force  of  gravity. 

The  standard  gravity  adopted  by  physicists  is  that  at  the  level  of 
the  sea  in  latitude  45°. 

Tables  of  corrections  for  gravity  are  given  in  Section  VIII.1 

103.  Reduction  to  sea  level. — It  was  mentioned  in  paragraph  5  that 
the  atmospheric  pressure  was  in  the  main  nothing  more  than  the 
weight  of  a  vertical  column  of  air  extending  to  the  limits  of  the  at- 

i  According  to  the  formula  for  the  force  of  gravity  adopted  by  the  International  Bureau,  of  Weights  and 
Measures  we  have  for  the  variations  in  gravity  due  to  the  latitude 
tt  0=046  (1-0.00259  cos  2  0) 

The  variation  in  the  force  of  gravity  for  different  altitudes  is  small,  and  is  given  by  the  formula 

G"h=00  (1-0.0000000597  h) 
h  being  the  elevation  in  feet. 

Neglecting  this  latter  factor,  the  application  of  the  gravity  correction  is  equivalent  to  multiplying  the 
height  of  the  barometer  by  the  factor 

(I -0.00259  cos  29) 


64        BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE 

mosphere.     It  naturally  follows  that  as  we  go  above  the  general  sur 
face  of  the  earth,  whether  in  balloons  or  by  ascending  mountains 
the  atmospheric  pressure  becomes  less  and  less  as  we  leave  more  anc 
more  of  the  air  beneath  us.     When  it  is  desired  to  chart  and  com 
pare  simultaneous  observations  of  atmospheric  pressure  over  extendec 
areas,  and  at  various  elevations  above  sea  level,  no  inference  can 
easily  be  drawn  from  the  actual  pressures  themselves,  but  each  must 
be  reduced  to  some  standard  level.     The  sea  level  plane  is  most  gen- 
erally adopted,  but  the  selection  of  a  plane  2,000  to  5,000  feet  above 
the  sea  offers  more  rational  conditions  in  certain  respects  and  is 
sometimes  advocated.     For  comparative  purposes,  therefore,  baro- 
metric observations  from  different  stations  require  a  "  reduction  for 
elevation." 

We  can  form  a  clear  idea  of  what  is  wanted  by  confining  our  atten- 
tion to  the  case  of  a  barometer  in  a  balloon  at  an  elevated  point 
above  the  sea.  The  reduction  for  elevation  is  simply  a  measure,  ex- 
pressed in  inches  of  the  mercurial  column,  of  the  weight  of  the  column 
of  air  between  the  balloon — that  is,  the  barometer  cistern — and  sea 
level.  '  This  weight  evidently  depends  not  only  upon  the  elevation 
above  sea,  but  also  upon  the  mean  temperature  of  the  air  below  the 
balloon  and  the  amount  of  moisture  it  contains. 

The  temperature  and  moisture  conditions  are  easily  conceived  of 
in  the  case  of  a  barometer  in  a  balloon  with  a  great  ocean  of  air 
directly  beneath,  but  when  we  consider  the  reduction  for  elevation  of 
barometric  observations  taken  over  extended  plateaus  and  at  great 
distances  from  sea  level,  such,  for  example,  as  the  reduction  of  obser- 
vations at  Denver,  Colo.,  no  clear  meaning  attaches  to  the  tempera- 
ture and  density  of  the  air  column;  in  fact,  the  air  column  can  not 
have  any  real  existence,  and  this  constitutes  a  considerable  difficulty 
in  computing  satisfactory  values  for  the  reductions  for  elevations. 
Approximate  values  only,  therefore,  are  possible.  Considerations 
such  as  these  lead  us  to  see  the  advantage  of  making  all  reductions  to  a 
plane,  say,  5,000  feet  above  sea  level,  in  which  case  an  air  column 
actually  exists,  and  has  a  definite  mean  temperature,  humidity,  etc. 

NOTE.— The  diminution  of  gravity  as  we  go  from  latitude  45°  to  the  Equator  causes  the  mercury  in  the 
barometer  to  weigh  less,  and  hence  for  a  given  pressure  in  the  atmosphere  the  mercury  in  the  barometer 
stands  higher  than  it  would  if  the  force  of  gravity  preserved  the  uniform  standard  value.  Therefore,  the 
farther  a  barometer  is  removed  from  latitude  45°  the  greater  its  correction  becomes,  so  that  at  the  Equator 
a  pressure  that  appears  to  be  30  inches  (at  sea  level)  is  really  about  29.92  inches. 

It  should  be  noticed  that  when  the  barometer  is  thus  corrected  for  its  peculiar  error  due  to  the  influence 
on  it  of  variations  of  gravity  the  pressure  that  it  then  gives  is  the  actual  pressure  of  the  air  at  each  latitude 
expressed  in  terms  of  an  absolute  and  not  a  variable  standard. 

It  is  important  to  remember  that  the  barometric  pressure  is  due  not  only  to  the  weight  of  the  air,  but 
also  to  the  prevailing  winds,  the  rapid  heating  or  cooling  and  consequent  expansion  or  contraction  of  low 
layers  of  air,  and  to  other  causes. . 

In  the  mercurial  barometer  we  balance  this  elastic  pressure  by  weight  of  quiescent  mercury;  a  change  of 
the  force  of  gravity  will  change  the  weight  of  the  column  of  mercury  without  necessarily  changing  the 
atmospheric  pressure. 

' 


BAROMETERS    AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        65 

Various  more  or  less  arbitrary  computations  of  the  temperature  to 
be  used  in  finding  the  reduction  for  elevation  have  been  tried  from 
time  to  time.  At  present  the  temperature  selected  for  the  reduction 
of  the  observations  of  the  Weather  Bureau  is  the  mean  of  the  current 
air  temperature  and  that  of  the  preceding  8  a.  m.  or  8  p.  m.  obser- 
vation. 

104.  Determination  of  height  by  barometric  readings. — The  calcula- 
tion of  elevation  above  sea  level  by  barometric  readings  involves  all 
the  principles  and  encounters  all  the  difficulties  of  the  "reduction  to 
sea  level"  described  above.     This  method,  therefore,  of  measuring 
heights  is  to  be  used  only  when  others  are  not  possible,  and  numerous 
observations  should  be  made  to  eliminate  the  very  large  accidental 
errors  to  which  the  method  is  subject. 

As  the  aneroid  barometer  is  used  by  tourists  and  others  so  largely 
in  connection  with  determination  of  elevation,  some  further  discussion 
of  the  subject  is  here  given. 

105.  Determination  of  heights  by  the  aneroid. — A  reading  of  a  barome- 
ter at  a  single  station,  without  reference  to  the  air  temperature  and 
corresponding  pressure  at  some  adjacent  points  whose  elevations  are 
known,  gives  only  the  crudest  possible  idea  of  the  elevation  of  the 
station,  and  the  neatly  constructed  little  pocket  aneroids  in  morocco 
cases  on  sale  in  the  shops,  having  their  dials  graduated  to  feet  of 
elevation  above  sea  level,  are  to  be  regarded  as  extremely  inaccurate, 
especially  if  the  scale  of  altitudes  is  graduated  upon  the  same  metal 
plate  as  the  scale  of  inches.     In  many  aneroids  the  scale  of  feet  is 
adjustable,  and  on  this  account  may  serve  to  some  advantage  for 
showing  small  differences  of  elevation,  but  at  the  best  all  direct  indi- 
cations of  elevation  from  readings  on  the  dial  of  the  aneroid  are  only 
roughly  approximate. 

106.  Adjustable  scale  of  elevations,  how  used. — If  the  scale  of  feet  is 
adjustable  on  an  aneroid  for  moderate  changes  of  elevation,  it  may 
be  conveniently  used  as  follows:  If  a  tourist  is  about  to  set  out  on  a 
short  expedition  to  an  elevated  point,  starting  at  a  station  of  which 
the  elevation  is  known,  let  him  set  the  movable  scale  of  the  aneroid 
so  that  the  proper  graduation,  marking  the  known  elevation,  stands 
opposite  the  index  hand.     On  reaching  the  elevated  point  the  position 
of  the  hand  on  the  scale  of  feet  will  now  indicate  approximately  the 
new  elevation,  provided,  of  course,  the  scale  has  not  been  shifted,  and 
provided  further  that  the  real  air  pressure  was  uniform  throughout 
the  vicinity  and  did  not  change  at  all  during  the  time  occupied  in  the 
expedition.     If  this  time  was  several  hours  or  the  distance  consider- 
able, the  result  may  be  very  greatly  in  error.     Suppose  we  know  our 
elevation  to  be  500  feet  and  we  set  the  scale  to  this  point.     To-morrow 

22937—12 5 


66        BAKOMETERS    AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

the  aneroid  may  indicate  only  200  or  even  800  feet,  the  new  value 
being  a  direct  result  simply  of  the  changes  in  air  pressure. 

A  variation  of  one-tenth  of  an  inch  in  the  barometric  pressure 
affects  an  observation  of  altitude  by  from  nearly  100  to  nearly  150 
feet,  according  (1)  to  the  elevation  itself  and  (2)  the  temperature  at 
the  time.  On  this  account  and  because  of  the  large  accidental  errors 
to  which  aneroids  are  subject,  as  described  above,  they  are  unreliable 
in  the  determination  of  elevations. 

107.  To  determine  heights  as  well  as  can  be  done  by  pressure  meas- 
urements, it  is  necessary  that  simultaneous  observations,  not  only  of 
the  barometer  but  of  the  temperature  and  humidity  of  the  air,  be  made 
at  one  or  more  adjacent  stations  of  which  the  elevations  are  known. 
These  observations,  by  means  of  suitable  tables,  will  give  the  differ- 
ence in  elevation  of  the  stations,  and  the  mean  result  from  a  large 
number  of  such  simultaneous  readings  will  give  a  fairly  accurate 
value  for  the  desired  elevation,  especially  if  the  differences  of  elevation 
and  the  distances  between  the  stations  are  not  very  great. 

108.  The  care  and  preservation  of  barometers. — A  barometer  is  a  very 
delicate  instrument,  and  in  general  must  be  handled  with  great  care ; 
therefore  observers  in  handling  a  barometer  should  first  inform  them- 
selves as  to  the  best  methods  to  follow  and  the  various  precautions  to 
observe,  as  embodied  in  the  instructions  given  below. 

109.  When  a  new 'barometer  is  received,  in  unpacking  it  should  be 
lifted  cistern  uppermost  from  the  box  and  all  wrappings  removed  after 
placing  the  barometer  in  a  horizontal  position.     When  moved  about, 
the  cistern  end  should  be  carried  uppermost. 

110.  To  turn  the  barometer  tube  end  up,  bring  it  first  gradually  to 
a  horizontal  position,  watching  for  a  small  bubble  at  the  cistern. 
This  should  never  be  very  large,  nor  should  it  be  absent,  in  which  case 
there  may  be  serious  pressure  from  within,  tending  to  force  the  mer- 
cury out  through  joints  of  the  cistern,  etc.     If  necessary,  the  adjusting 
screw  should  be  turned  so  that  the  bubble  is  not  larger  than  a  space 
within  which  a  10-cent  coin  could  be  placed.     The  tube  may  then  be 
gradually  elevated  to  an  upright  position.     The  mercurial  column 
should  not  be  lowered  until  the  instrument  is  safely  suspended  from 
a  hook. 

111.  Never  remove  a  good  barometer  from  its  supports  while  the 
mercurial  column  is  at  or  near  its  normal  height.     Always  screw  up 
the  cistern  until  the  top  of  the  column  is  just  visible  at  the  top  of  the 
opening  in  the  brass  case.     Do  not  subject  the  barometer  to  quick 
movements  or  sudden  changes  in  its  position;  always  move  it  about 
slowly  and  regularly  and  change  its  position  gradually.     Do  not 
handle  or  carry  the  barometer  in  an  upright  position.     Handle  it 
horizontally,  or  upside  down  as  far  as  possible,  preferably  the  latter. 
The  proper  procedure  to  invert  a  barometer  is  as  follows : 


BAROMETERS   AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        67 

Examine  the  cistern  to  see  if  there  is  any  special  air  vent  as  at  d, 
figure  9,  in  the  Tuch  cistern.  Screw  up  the  mercury  until  it  reaches 
the  top  of  the  cistern.  Then  close  tightly  the  air  vent,  and  continue 
screwing  up  the  cistern  until  the  top  of  the  mercurial  column  reaches 
the  summit  of  the  opening  in  the  metal  tube.  Always  avoid  screwing 
up  the  cistern  until  the  tube  is  entirely  filled  with  mercury.  It  is 
impossible  to  tell  exactly  when  the  tube  is  full,  and  a  turn  too  much 
of  the  screw  is  almost  sure  to  force  the  mercury  through  the  joints 
of  the  cistern  or  even  the  pores  of  the  leather  bag  and  lead  to  very 
serious  injury  of  the  barometer.  Do  not  strain  the  screw  if  it  goes 
hard.  Mercury  may  have  leaked  from  the  cistern,  and  what  remains 
be  insufficient  to  fill  the  tube.  A  barometer  can  be  safely  inverted 
even  if  there  is  quite  a  deficient  supply  of  mercury  in  the  cistern. 

When  the  mercury  is  near  the  top  of  the  tube,  remove  the  barome- 
ter from  its  supports  and  incline  slowly,  listening,  meanwhile,  for  any 
slight  sound  or  "click"  that  may  be  emitted  from  the  top  of  the 
barometer.  When  the  tube  is  nearly  horizontal,  watch  for  the 
appearance  of  an  air  bubble  at  the  cistern  end,  showing  there  is  still 
a  small  free  space  within.  From  the  horizontal  position  the  instru- 
ment may  be  turned  cistern  end  up  without  any  special  precautions 
and  may  then  be  handled  and  carried  with  ease  and  safety.  It  is 
even  advisable  now  to  loosen  the  cistern  screw  a  turn  or  so,  that  there 
may  be  plenty  of  free  space  in  the  cistern. 

112.  Special  instructions  for  handling  marine  barometers. — It  will  be 
readily  understood  from  the  description  of  the  marine  barometer  and 
its  cistern  that  special  care  must  be  employed  in  inverting  such  a 
barometer.     In  this  case  the  cistern  is  not  and  can  not  be  filled  with 
mercury.     Moreover,  the  constriction  of  the  tube  prevents  the  vac- 
uum from  filling  quickly,  so  that  to  invert  a  barometer  of  this  kind 
the  tube  must  be  inclined  30°  or  40°  from  the  vertical  and  held  in  such 
a  position,  and  farther  inclined,  if  necessary,  until  the  vacuum  cham- 
ber is  completely  filled  with  mercury,  whereupon  the  instrument  may 
be  full}'  inverted  and  handled  without  danger,  cistern  uppermost. 

Since  the  cistern  is  only  partly  full  ordinarily,  the  marine  barometer 
is  more  liable  to  injury  in  shipment  than  other  barometers  and,  in 
fact,  is  best  transported  by  hand. 

113.  The  "metallic  click." — The  so-called  " metallic  click"  is  best 
produced  while  the  barometer  is  inclined  at  about  45°,  or  possibly 
still  more  nearly  horizontal  at  high-level  stations.     The  cistern  must 
not  be  screwed  up  too  much.     The  "click"  occurs  just  as  the  mer- 
cury moving  up  the  tube  reaches  the  top  and  completely  fills  it.     If 
the  barometer  is  quickly  inclined,  the  violent  shock  of  the  mercury 
against  the  top  of  the  tube  is  sometimes  sufficient  to  crack  the  tube. 
Hence,  sudden  movements  of  this  sort  are  always  attended  with 
danger  to  the  barometer. 


68        BAROMETERS   AND  MEASUREMENT   OF   ATMOSPHERIC    PRESSURE. 

Many  think  they  can  judge  of  the  excellence  of  the  vacuum  in  a 
barometer  by  the  character  of  the  "metallic  click."  It  is  exceedingly 
deceptive,  however,  and  even  experts  are  able  to  draw  only  approxi- 
mately correct  conclusions  from  its  character.  The  greatest  caution 
should  be  exercised  in  producing  the  click,  as,  if  the  vacuum  is  first 
class,  it  tends  to  injure  the  barometer.  A  good  plan  is  to  incline  the 
barometer,  as  described  above,  until  the  mercury  almost  reaches  the 
top  of  the  tube;  then,  holding  it  in  this  position,  move  it  somewhat 
quickly,  but  very  slightly  and  regularly,  back  and  forth  three  or  four 
times  exactly  in  the  direction  of  its  length,  and,  if  necessary,  changing 
the  angle  of  inclination  and  increasing,  very  cautiously,  the  intensity 
of  the  shaking  motion  until  two  or  three  gentle  clicks  may  be  heard. 
Too  great  care  can  not  be  exercised  in  this  respect,  and  only  the  most 
gentle  clicks  should  be  produced.  Even  then,  with  very  perfect 
vacua,  the  internal  stress  is  very  great,  and  barometer  tubes  that 
have  been  subjected  to  boiling  in  the  process  of  filling  and  are  not 
thoroughly  annealed  are  sometimes  in  such  a  state  of  internal  stress 
as  to  be  very  easily  cracked  and  injured. 

114.  Handling  barometers  at  elevated  stations. — In  the  case  of  sta- 
tions from  3,000  to  10,000  feet  or  more  above  sea  level  the  top  of  tha 
mercurial  column,  in  extreme  cases,  is  a  long  distance  from  the  top 
of  the  tube.     It  is  not  advisable,  therefore,  when  it  is  desired  to  invert 
such  a  barometer,  to  screw  up  the  cistern  immediately  until  the  col- 
umn reaches  nearly  the  top  of  the  tube.     A  better  plan  is  to  raise  the 
column  only  2  or  3  inches,  then,  while  gradually  inclining  the  instru- 
ment, continue  to  screw  up  the  cistern  until  the  column  is  about  to 
disappear  from  view  at  the  top.     The  object  of  this  is  to  avoid  sub- 
jecting the  cistern  to  the  considerable  hydrostatic  pressure  that  occurs 
if  the  column  is  raised  several  inches  above  that  which  the  air  pressure 
itself  is  capable  of  supporting. 

At  an  elevated  station  the  barometer  must  be  in  a  much  more  nearly 
horizontal  position  to  produce  the  "metallic  click"  than  at  sea  level. 

115.  The  best  possible  care  a  barometer  can  receive  is  to  be  pro- 
tected from  accumulations  of  dust,  etc.,  and  left  quite' alone.     When 
readings  must  be  taken,  and  the  barometer  is  suspended  from  a  hook 
upon  which  it  is  drawn  out  to  a  position 'convenient  for  reading,  the 
rough  sliding  of  the  barometer  along  the  hook,  together  with  the 
springing  movement  up  and  down,  and  finally  the  knocks  the  cistern 
is  apt  to  receive  when  the  instrument  is  returned  to  the  box,  are  all 
very  injurious  to  the  condition  of  the  barometer  and  are  to  be  avoided 
by  gentle  and  careful  handling. 

116.  The  results  of  comparative  barometer  readings  conclusively 
show  that  in  spite  of  every  care  a  difference  of  several  thousandths  of 
an  inch  in  the  indications  of  two  or  more  instruments  can  not  ordi- 
narily be  avoided.     Any  change  of  instruments  at  a  station  therefore 


BAROMETERS    AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        69 

is  apt  to  make  a  more  or  less  objectionable  break  in  the  strict  con- 
tinuity of  the  pressure  reports  from  that  station  and  obviously  such 
changes  should  be  made  only  when  absolutely  necessary. 

117.  After  continued  use  the  mercury  in  the  cistern  of  a  barometer 
loses  its  brilliant  surface  and  becomes  coated  with  a  slight  film  of 
oxide.     This  does  not  impair  the  barometer  to  any  serious  extent, 
and  very  accurate  readings  can  yet  be  made.     It  is  a  bad  practice 
to  clean  the  mercury  in  barometers  as  s6on  as  it  becomes  slightly 
dull  and  tarnished.     Leaks  are  apt  to  be  started  in  the  joints  of  the 
cistern,  and  slight  changes  in  the  position  of  the  ivory  point  give 
rise  to  new  and  unknown  corrections  for  instrumental  error.     The 
mercury  itself  is  apt  to  become  contaminated  with  impurities  and 
afterwards  will  remain  bright  only  a  very  short  tune. 

118.  Comparative  barometer  readings. — Each  regular  Weather  Bu- 
reau station  is  supplied  with  two  good  barometers  to  lessen  the  chances 
of  a  break  in  the  record  and  to  guard  against  erroneous  reports  from 
the  use  of  imperfect  instruments.     Monthly  therefore,  and  on  other 
special  occasions,  as  further  specified  hi  paragraph  122,  five  com- 
parative readings  of  all  barometers  on  station  should  be  made  at 
uniform  intervals  of  hours,  half  hours,  or  quarter  hours,  as  may  be 
most  convenient  to  the  observer. 

119.  As  the  object  of  the  comparative  readings  is  to  ascertain 
accurately  the  amount  of  discordance  between  the  barometers  and 
enable  the  main  office  to  replace  defective  instruments,  it  is  impor- 
tant that  the  observer  use  more  than  ordinary  care  to  read  the  barome- 
ters exactly  as  they  are.     He  should  not  feel  biased  or  disposed,  in 
the  slightest  degree,  to  make  the  readings  come  out  one  way  more 
than  another.     His  whole  endeavor  should  be  to  make  the  settings 
and  readings  as  accurately  as  possible,  without  any  regard  as  to 
how   the  readings  may  differ  in  the  end.     When  the  pressure  is 
found  to  vary  rapidly,  make  the  readings  of  the  two  or  more  instru- 
ments as  quickly  as  possible,  and  throughout  the  series  endeavor  to 
keep  the  temperature  stationary. 

120.  Before  each  reading  the  cistern  of  the  barometer  should  be 
unscrewed  so  as  to  lower  the  mercury  one-sixteenth  to  one-eighth  of 
an  inch  below  the  ivory  point  and  the  setting  then  carefully  made. 

121.  An  interval  of  two  or  tnree  hours  should  intervene  after 
barometers  are  unpacked,  cleaned,  or  moved  to  a  new  office  and 
hung  in  position  before  comparative  readings  are  commenced. 

\'2'2.  Comparative  readings  should  always  be  made  with  new 
barometers  whenever  received  at  a  station,  and  also  both  before 
and  after  instruments  are  removed  from  one  location  to  another  or 
cleaned. 

123.  Suggestions  and  instructions  for  cleaning  barometers. — In  a  few 
cases  observers  are  authorized  to  clean  cisterns  of  barometers  that 


70        BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

are  very  dirty  and  can  not  easily  be  replaced.  (See  paragraph  187.) 
The  following  instructions  will  then  guide  in  the  proper  performance 
of  the  work: 

124.  Take  a  series  of  five  comparative  readings  before  the  work  is 
begun. 

125.  Provide  one  or  more  very  clean,  dry  porcelain  or  glass  cups 
or  saucers.     Avoid   the   use   of   damp,   unclean,   or   metal   vessels. 
Cleanse  the  vessels  by  thorough  washing  in  soap   and  water  and 
wipe  dry  with  a  clean  towel,  finally  polishing  the  vessel  with  tissue 
or  similar  soft  paper.     Provide,    also,   some  pieces   of  clean   cloth 
and  sheets  of  tissue  paper  for  cleansing  the  glass  parts  of  the  cistern, 
also  a  few  small  sheets  of  clean  white  paper  about  4  by  6  inches 
for  use  in  filtering  the  mercury.     Calendered   letter  paper  is  not 
so  good  as  the  ordinary  so-called  book  paper.     A  most  convenient 
position  for  cleaning  a  barometer  is  to  be  seated  in  front  of  a  desk 
with  a  drawer  at   the   top   and  side  partly  opened.     This   affords 
convenient  corners  in  which  the  barometer  can  be  rested  in  upright 
positions  during  the  process. 

126.  The   barometer  will  be   removed   from   its   box   or  support 
and  inverted,  as  described  in  paragraph  111. 

Unscrew  with  one  hand  the  portion  of  the  cistern  marked  S, 
figure  3,  grasping  with  the  other  hand  only  the  narrow  flange  R.1 

127.  Next  separate  the  two  wooden  portions  of  the  cistern  marked 
i  and  j  by  loosening  the  four  screws  uniting  the  split-ring  clamp 
marked  Z  and  M,  in  figure  3.     It  is  important  that  each  screw  be 
loosened  a  little  in  turn,  otherwise  an  uneven  strain  may  be  thrown 
upon  some  portion  of  the  fragile  wooden  flange  and  chip  out  a  piece. 
After  loosening  each  of  the  screws  one  may  be  taken  out  entirely, 
and  the  whole  system  of  split  rings  still  interlocked  by  the  screws 
will  generally  unfold  from  around  the  cistern.     Sometimes  another 
screw  must  be  taken  out. 

If  the  rings  are  separated,  they  should  afterwards  be  united  again 
precisely  in  the  original  relation.  When  removing  the  w(  oden  piece 
j,  to  which  the  leather  bag  is  attached,  lift  it  cautiously  directly  up 
from  the  part  i  so  as  not  to  spill  the  mercury,  which  is  thereby 
exposed  and  should  just  about  fill  i.  Hold  a  clean,  dry  vessel  close 
under  the  flange  of  i  and  pour  out  steadily  from  the  cistern  all  or 
nearly  all,  the  mercury  it  contains.  The  mercury  will  not  leave  the 
open  end  of  the  barometer  tube  so  long  as  the  latter  is  not  raised 

1  If  mercury  has  leaked  out  of  the  cistern  this  will  generally  be  indicated  by  the  presence  of  minute 
globules  of  mercury  adhering  to  the  threads  of  the  screw  O.  In  such  a  case  the  cap  at  the  extreme  bottom 
of  the  cistern  should  be  unscrewed,  instead  of  the  portion  marked  8,  thereby  preventing  the  escape  of 
the  mercury  which  has  leaked  out.  As  this  mercury,  by  reason  of  its  contact  with  the  metal  parts  of 
the  cistern,  is  impure,  it  must  be  emptied  separately,  and  under  no  circumstances  afterwards  used  in  the 
barometer  or  mixed  with  good  mercury,  as  the  whole  will  be  rendered  impure. 

To  empty  thi?  impure  mercury  from  the  cistern  the  finger  must  be  used  to  force  the  kid-skin  bag  up 
into  the  cistern  while  the  barometer  is  inclined  and  the  impure  mercury  poured  out. 


BAROMETERS   AND  MEASUREMENT   OF    ATMOSPHERIC    PRESSURE.        71 

much  above  a  horizontal  position,  and  generally  not  then  unless  the 
opening  is  large  and  the  tube  shaken  or  jerked  a  little.  Care  must 
be  taken  to  prevent  the  mercury  from  passing  out  of  the  tube.  The 
barometer  is  then  returned  to  its  inverted  position  and  the  remaining 
parts  of  the  cistern  removed  by  loosening  the  screws  P  and  P;  here 
again  each  screw  must  be  loosened  a  little  in  turn  to  avoid  chipping  or 
cracking  the  glass  cylinder.  If  a  small  globule  of  mercury  remains 
in  the  glass  cistern,  allow  the  latter  to  rest  in  its  position,  while  the 
boxwood  piece,  i,  the  metal  flange,  R,  and  the  screws,  P,  are  removed. 
Then  holding  the  glass  cylinder  in  position  with  the  fingers,  empty 
what  remains  of  the  mercury  in  the  cistern.  In  handling  the  little 
leather  washers  taken  from  the  parts  of  the  cistern,  avoid  wrinkling 
or  creasing  them  or  otherwise  changing  their  form,  as  any  injury  of 
this  kind  will  probably  result  in  leaks  that  can  not  be  prevented 
except  by  new  washers. 

128.  The  barometer  tube  and  attached  wooden  piece  G,  figure  3,may 
be  next  withdrawn  from  the  metal  sheath  and  all  the  parts  thoroughly 
cleaned.     Before  removing  the  tube  notice  exactly  the  position  of  the 
ivory  point  in  reference  to  the  outside  sheath  so  that  it  may  be 
returned  to  this  position,  otherwise  a  change  may  be  introduced  in 
the  correction  for  instrumental  error.     In  all  probability  small  quan- 
tities of  mercury  will  be  spilled  into  or  remain  in  various  little  cracks 
and  crevices  while  the  cistern  is  being  emptied.     These,  by  all  means, 
should  be  thoroughly  dislodged,  especially  from  about  the  metal  parts. 
With  the  glass  tube  removed,  the  sheath  should  be  tapped  and  shaken 
smartly  to  remove  all  small  globules  of  mercury.     It  may  then  be 
wiped    and   cleaned  thoroughly   with  cloths  or  chamois  skin.     In 
case  the  scale  is  somewhat  dull  and  tarnished  it  may  be  brightened 
by  suitable  polishing,  but  this  is  a  delicate  operation  and  should  be 
avoided  rather  than  otherwise.     The  danger  lies  in  shifting  the  posi- 
tion of  the  scale,  and  if  polishing  is  absolutely  necessary,  it  should, 
therefore,  be  done  with  very  great  care. 

The  upper  portion  of  the  glass  tube  should  also  be  cleansed  on  the 
outside  with  the  aid  of  a  damp  cloth  if  necessary. 

129.  Air  in  barometer  tubes. — How  air  can  gam  entrance  to  the 
vacuum  of  a  barometer  otherwise  in  good  condition,  which  is  sup- 
posed to  have  been  hanging  quietly  and  undisturbed  upon  its  sup- 
ports, is  a  matter  that  is  very  difficult  both  to  imagine  and  to  explain. 
No   case  of     this  sort  has  ever  occurred  among  the  hundreds  of 
barometers  handled  at  this  office,  so  that  when  such  a  defect  is  dis- 
covered in  an  instrument  in  use  at  a  station  the  observer  in  investi- 
gating the  cause  and  reporting  the  matter  should  make  sure  that 
the  barometer  has  not  been  tampered  with  or  roughly  handled  by 
unauthorized  persons,  as,  if  uninjured  in  other  respects,  misusage  is 
the  most  probable  explanation  of  the  defect. 


72        BAROMETERS   AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

130.  If  an  appreciable  quantity  of  air  is  in  the  tube  at  the  time  of 
cleaning,  it  can  be  seen  more  or  less  conspicuously  in  the  shape  of  a 
small  bubble  or  bubbles  adhering  closely  to  the  walls  of  the  tube. 
If  these  bubbles  appear  no  larger  than  good-sized  pinheads,  and 
especially  if  they  are  not  more  than  halfway  up  the  tube,  then  it  is 
certain  that  the  condition  of  the  vacuum  is  more  likely  to  be  greatly 
impaired  than  improved  by  attempts  to  remove  them. 

131.  Sometimes  the  barometers  that  observers  may  be  called  upon 
by  private  parties  or  friends  to  inspect  or  repair,  seem  to  have  numer- 
ous rather  flat-shaped  air  bubbles  firmly  lodged  against  the  sides  of 
the  tube.     Generally  these  are  not  air  bubbles  at  all,  but  are  parti- 
cles of  moisture,  the  presence  of  which  is  due  to  carelessness  in  the 
original  preparation  and  filling  of  the  tube.     The  edges  of  an  air 
bubble  are  sharp  and  the  mercury  generally  remains  bright  and 
makes  well-defined  contact  at  a  steep  angle  with  the  glass.     If  some 
moisture  is  present,  either  alone  or  with  the  air,  the  edges  are  less 
clearly  defined,  the  mercury  is  oxidized,  and  the  angle  of  contact  is 
less  steep,  the  bubble  itself  being  very  flat. 

It  is  impossible,  without  entire  cleansing,  drying,  and  refilling,  to 
do  anything  with  a  barometer  that  contains  moisture. 

132.  If  a  bubble  or  so  of  air  is  present  in  a  tube,  the  plan  that 
should  first  be  tried  to  remove  it  is  as  follows : 

First  method. — Incline  the  tube  45°  or  thereabouts,  with  the  open 
end  up,  and  tap  it  gently  in  the  vicinity  of  the  bubble,  revolving  the 
tube  a  little  at  the  same  time  so  as  to  encourage  the  bubble  to  creep 
along  the  inclined  surface  of  the  glass.  If  the  inclination  is  too 
great  the  bubble  will  be  greatly  compressed  by  the  weight  of  mercury 
above  it;  if  too  small  the  bubble  will  not  tend  to  move. 

If  the  treatment  is  successful  and  the  bubble  is  removed,  the  result 
will  probably  be  beneficial;  but  at  best  the  operation  is  generally  very 
tedious,  and  often  the  bubble  seems  to  grow  smaller  and  finally  dis- 
appears, being  separated  into  almost  imperceptible  portions  which 
remain  distributed  along  the  walls  of  the  tube. 

Second  method. — The  following  plan  is  more  frequently  applied, 
especially  when  the  quantity  of  air  already  in  the  tube  is  considerable, 
is  lodged  at  the  top,  and  must  be  partially  removed  at  least : 

Empty  an  inch  or  two  of  mercury  from  the  tube.  Close  the 
open  end  tightly  with  the  gloved  finger  and  cause  a  large  bubble  of 
air  to  glide  slowly  and  regularly  along  the  tube  until  it  unites  with 
all  the  portions  of  air  it  is  desired  to  remove.  The  large  bubble  is 
then  as  slowly  and  gradually  worked  to  the  open  end  of  the  tube 
again,  using  every  possible  precaution  to  prevent  small  portions  of 
the  bubble  from  separating  and  remaining  behind.  Such  a  bubble 
of  air  may  sometimes  be  successfully  passed  once  into  and  out  of  the 
tube,  but  even  at  the  best  the  vacuum  in  a  barometer  that  has  been 


BAROMETERS    AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        73 

treated  in  this  manner  is  very  apt  to  be  greatly  impaired  and  can  not 
be  restored.  The  reason  of  this  is  that  the  glass  walls  of  the  tube 
have  very  strong  hygroscopic  properties,  and  while  the  air  bubble  is 
passing  along  the  tube  considerable  portions  of  both  moisture  and  air 
are  invisibly  retained  upon  the  walls  of  the  tube.  While,  therefore, 
a  bubble  of  air  may  be  successfully  passed  once  into  and  out  of  the 
tube,  a  repetition  will  be  attended  with  less  good  effect,  as  in  the 
meantime  the  moisture  and  gases  of  the  bubble  will  have  acted  upon 
the  mercury  to  produce  oxidized  films  that  will  probably  adhere  to 
the  walls  of  the  tube,  so  that  when  bubbles  are  again  passed  there  will 
presently  be  a  marked  tendency  to  cling  to  the  tube  and  leave 
small  detached  bubbles  imprisoned  against  the  walls.  When,  after- 
wards, the  barometer  is  set  up  the  walls  in  the  upper  portion  and 
near  the  vacuum,  being  no  longer  subjected  to  the  full  air  pressure  as 
they  were  while  the  bubble  was  passing  along  the  tube,  now  readily 
give  off  both  air  and  moisture,  and  in  many  cases  numerous  little 
bubbles  form  against  the  walls  even  below  the  top  of  the  column  and 
probably  later  work  their  way  into  the  vacuum. 

133.  The  removal  of  air  from  a  barometer  tube,  therefore,  can  not 
be  perfectly  effected  in  any  such  way,  and  should  not  be  undertaken 
unless  the  defect  is  a  very  serious  one.     If  the  comparative  readings 
taken  before  cleaning  a  barometer  do  not  show  serious  errors,  any  air 
the  tube  may  be  thought  to  contain  had  best  be  allowed  to  remain. 

134.  One  of  the  most  difficult  and  delicate  parts  of  the  process  of 
cleaning  is  that  about  the  wooden  piece,  G,  and  ivory  point.     The  deep 
and  narrow  annular  space  between  the  glass  tube  and  the  boxwood  is 
generally  covered  with  oxide  of  mercury,  which  should  be  thoroughly 
removed  by  repeated  wiping  with  clean  cloths  applied  upon  the 
ends  of  slender  sticks  or  by  similar  means.     Tufts  of  raw  cotton 
will  adhere  firmly  to,  and  are  readily  wrapped  about,  rough  sticks 
and  may  serve  with  advantage  in  wiping  out  the  narrow  spaces. 
Sometimes,  however,  the  space  is  so  small  that  it  can  not  be  properly 
cleaned.     Care  must  be  observed  not  only  here  but  in  subsequent 
operations,  to  blow  away  or  otherwise  remove  every  vestige  of  lint, 
dust,  shreds  of  cotton,  etc.,  as,  if  allowed  to  remain  about  the  parts 
of  the  cistern,  they  will  quickly  find  their  way  to  the  surface  of  the 
mercury,  upon  which  they  will  float  about  to  the  detriment  of  accu- 
rate adjustments. 

It  is  obvious  that  the  delicate  ivory  point  should  be  handled  with 
great  care. 

135.  The  glass  cy Under  of  the  cistern  should  be  washed  hi  soap 
and  water  and  thoroughly  rinsed  in  copious  applications  of  fresh 
water.     After  this  it  should  not  be  touched  with  unprotected  hands» 
especially  upon  the  inside.     Wipe  it  thoroughly  dry  with  a  clean,  dry 
towel  or  handkerchief^  and   polish  with  clean  tissue  paper.     The 


74        BAROMETERS   AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

. 

remaining  wooden  portions  of  the  cistern  should  also  be  wiped 
thoroughly  clean  and  dry  without  touching  the  inside  with  the  bare 
fingers.  Shake  out  of  the  bag  as  far  as  possible  every  little  particle 
of  mercury  that  tends  to  remain  in  hidden  corners  and  crevices. 
These  little  particles  are  very  apt  to  be  dirty  and  impure,  and  should, 
therefore,  be  removed. 

136.  The  several  parts  of  the  barometer  should  be  replaced  in  the 
following  order: 

First,  return  the  glass  tube  to  its  sheath,  being  careful  to  place 
the  ivory  point  in  the  position  in  relation  to  the  scale,  or  front  of  the 
barometer,  formerly  occupied;  also  to  avoid  handling  the  end  portion 
of  the  barometer  tube  where  it  dips  into  the  cistern  with  the  bare 
fingers,  as  a  slight  film  of  oil  may  be  communicated  to  the  mercury 
of  the  cistern  by  this  means. 

The  glass  cylinder,  with  its  leather  washers,  one  at  each  end,  is 
next  placed  in  position,  followed  by  the  wooden  piece,  i,  and  the 
metal  flange  ring,  R.  The  three  long  screws,  P,  are  next  to  be 
'inserted  and  partially  screwed  up.  While  these  various  pieces  are 
still  loosely  held  by  the  screws,  it  is  well  to  jostle  the  parts  about 
a  little  and  twist  the  ring  and  boxwood  pieces  upon  each  other  and 
the  glass  cylinder.  In  other  words,  try  to  bring  the  surfaces  in  the 
several  joints  nicely  and  uniformly  into  contact  with  each  other,  and 
adjust  the  ring,  R,  so  that  the  screws  are  not  even  imperceptibly 
askew,  but,  when  properly  drawn  up,  produce  a  direct,  uniformly 
distributed  pressure.  When  the  parts  are  thus  adjusted  the  screws, 
P,  are  to  be  tightened  little  ~by  little,  each  one  a  little  in  turn  after  the 
others,  until  all  are  drawn  down  together  equally  tight.  The  Observer 
must  judge  of  this  partly  by  the  amount  he  has  turned  each 
screw  and  partly  by  the  resistance  it  offers  to  further  turning.  It 
is  not  necessary  that  the  screws  be  very  tight.  A  judicious  regard 
for  these  ideas  constitutes  in  part  the  skill  of  the  expert  and  is  the 
secret  of  perfect  joints.  To  disregard  them  produces  leaky  joints 
and  unequal  pressures  that  are  apt  to  break  the  fragile  boxwood 
flanges  or  crack  the  glass. 

Before  describing  the  filling  of  the  cistern,  some  tests  and  experi- 
ments showing  the  purity  and  properties  of  mercury  will  be  men- 
tioned. 

137.  Purity  of  mercury,  Jiow  tested.— Pure  mercury  is  beautifully 
briUiant  and  mobile,  and  does  not  exhibit  the  slightest  adhesion  to 
clean,  dry  glass  or  porcelain  surfaces,  whereas  the  amalgamation  of 
the  mercury  with  the  slightest  perceptible  traces  of  foreign  substances, 
such  as  lead,  tin,  zinc,  etc.,  changes  completely  the  character  of  this 
peculiar  substance.     Each  observer  should  try  for  himself  the  fol- 
lowing instructive  and  simple  experiment : 

. 
" 


BAROMETERS   AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        75 

Prepare  a  small,  shallow,  flat-bottomed  porcelain  cup  or  white  piece 
of  chinaware,  or  glass  vessel  if  the  others  are  not  to  be  had.  Wash 
and  dry  thoroughly  without  touching  the  inside  with  the  bare  fingers. 
The  vessel  may  be  just  a  little  warm  with  advantage.  Filter  into  the 
vessel,  through  a  paper  funnel,  such  as  described  below,  rather  less 
than  a  teaspoonful  of  pure  mercury.  If  the  mercury  has  been  prop- 
erly filtered  and  is  of  extreme  puritjr,  the  brilliant  globule  will  roll 
about  the  cup  with  the  greatest  activity,  as  the  latter  is  moved  a 
little,  and  will  draw  out  momentarily  into  slender  cylindrical  por- 
tions which,  if  broken  asunder,  will  quickly  separate  into  smaller 
portions,  which  draw  themselves  up  into  beautiful  little  spheres  or 
larger  rounded  buttons,  none  of  which  clings  in  the  slightest  degree 
to  the  clean  surface  of  the  vessel.  Under  favorable  conditions  and 
during  the  rapid  movements  of  the  mercury  a  scarcely  audible  but 
still  a  very  characteristic  crackling  sound  can  be  heard,  due  to  the 
development  of  small  sparks  of  electricity.  Such  is  the  character- 
istic behavior  of  clean,  pure  mercury  hi  a  clean  porcelain  dish.  If, 
however,  the  mercury  contains  the  most  minute  trace  of  lead,  tin, 
zinc,  etc.,  this  fact  wih1  be  shown  by  a  more  or  less  marked  tendency  of 
the  little,  slender  portions  of  the  mercury  to  draw  out  into  sharply 
pointed,  tapering  "tails,"  the  tip  ends  of  which  cling  to  the  vessel 
and  remain.  If  the  observer  is  not  in  possession  of  the  small  quan- 
tity of  extremely  pure  mercury  needed  in  the  above  experiment,  the 
most  striking  part  of  it  will  be  lost.  After  watching  the  beautiful 
manner  in  which  the  pure  mercury  rolls  about  the  dish,  add  to  it  a 
small  flake  of  lead  or  solder.  The  flake  should  be  a  very  small 
shaving  cut  off  with  a  penknife,  and  should  contain  not  nearly  so 
much  material  as  in  the  head  of  the  smallest  phi.  Place  this  upon 
the  mercury  and  allow  it  to  remain  a  moment.  It  will  presently  be 
wholly  dissolved.  Now  repeat  the  rolling  about  of  the  mercury  in 
the  cup  and  observe  the  wonderful  change. 

The  former  brilliant  globule  has  now  a  dull  surface,  with  its  edges 
clinging  at  many  points  to  the  surface  of  the  dish.  The  clean  white 
surface  of  the  dish  will  now  be  soiled  and  discolored  when  the  mer- 
cury is  made  to  flow  over  it  a  few  times.  The  presence  of  one  part 
of  lead  hi  one  hundred  thousand  parts  of  mercury  is  readily  shown 
by  this  test. 

Only  one  who  has  performed  this  experiment  is  prepared  to  appre- 
ciate fully  the  importance  of  absolute  cleanliness  in  barometer  cis- 
terns and  the  necessity  for  the  avoidance  of  the  slightest  metallic 
contamination  of  the  mercury. 

138.  Of  course,  the  mercury  used  in  the  above  experiment  can  not 
be  again  used  until  purified.  This  can  be  done  quite  well  by  washing 
with  dilute  nitric  acid,  about  one  volume  of  acid  in  fifteen  volumes 
of  water.  The  mercury  and  acid  may  be  placed  in  a  bottle  and  vi<>- 


76        BAEOMETEES   AND  MEASUBEMENT    OF   ATMOSPHEEIC    PEESSUEE. 

lently  shaken,  or  the  acid  may  be  poured  over  the  mercury  and 
allowed  to  remain  several  hours.  When  the  acid  has  thoroughly 
cleansed  the  mercury,  the  latter,  upon  the  contents  of  the  bottle 
being  violently  shaken,  will  break  up  into  very  fine  globules  which, 
for  a  moment,  do  not  coalesce.  This  formation  of  the  mercury 
into  minute  globules  hi  the  presence  of  dilute  acid  will  take  place 
only  with  quite  pure  mercury. 

139.  Returning  now  to  the  processes  of  restoring  the  barometer, 
the  next  step  is  to  filter  the  mercury  and  fill  up  the  cistern.     Roll  up 
a  small  sheet  of  clean  paper  into  a  sharp  cone,  looking  through  it  to 
the  light  to  see  that  the  opening  is  very  small.     Holding  the  cone  over 
a  clean  vessel,  partly  fill  it  with  mercury.     By  twisting  at  the  folds  of 
the  cone  hi  a  manner  that  the  observer  must  learn  by  trial,  the  open- 
ing at  the  point  may  be  regulated  to  any  size'desired,  even  while  the 
cone  contains  mercury.     Keep  the  cone  well  filled  with  mercury  until 
all  has  been  added,  and  do  not  allow  the  very  last  portion  to  pass 
through  the  filter.     If  the  observer  has  only  the  supply  of  pure  mer- 
cury taken  from  the  barometer,  economy  must  be  exercised,  but  there 
is  no  difficulty  whatever  in  being  able  to  filter  and  utilize  the  entire 
quantity  of  mercury  originally  in  the  barometer,  and  this  is  suffi- 
cient.    The  purity  of  some  of  the  filtered  mercury  may  be  tested  as 
described  above.     Another  indication  of  the^  purity  of  the  mercury 
is  the  character  of  the  mark  left  on  the  paper  cone  after  filtering. 
To  be  able  to  judge  by  this,  observers  must  filter  both  pure  and 
impure  mercury  and  compare  the  marks. 

140.  The  mercury  for  the  cistern,  having  been  filtered  at  least  once, 
may  next  be  filtered  into  the  cistern,  directing  the  little  stream  so  as 
to  strike  against  the  glass  cistern  to  avoid  inclosing  small  air  bubbles 
near  or  upon  the  barometer  tube.     The  open  end  of  the  tube  should, 
in  the  meantime,  be  completely  filled,  and  the  mercury  heaped  into 
a  little  button  on  the  tip  end .     This  button  will  unite  with  the  mer- 
cury of  the  cistern  as  it  rises  around  the  tube,  and  the  chances  of 
inclosing  air  in  the  tip  end  of  the  tube  are  thus  greatly  lessened. 

In  general,  the  cistern  should  be  filled  to  the  brim  of  the  piece,  i. 
Before  fitting  the  piece,  j,  the  leather  bag  should  be  pushed  out  from 
the  inside  and  every  effort  used  to  detach  and  remove  all  dust,  shreds, 
little  particles  of  leather,  etc. 

141.  In  securing  the  clamp  rings  the  screws  should  be  tightened  a 
little  at  a  tune,  and  the  precautions  cited  in  paragraph  136  observed 
to  insure  a  closely  fitting  and  uniformly  tight  joint. 

When  the  screws  are  all  tightened,  the  leather  bag  should  be  thrust 
up  into  the  wooden  piece,  j,  and  held  there  firmly  by  the  finger  while 
the  barometer  is  gradually  turned  right  side  up,  watching  to  see  if 
any  leaks  show  themselves  at  any  of  the  joints.  The  mercurial 
column  should  not  be  lowered  under  any  circumstances  at  this  time. 


BAROMETERS   AND  MEASUREMENT   OF   ATMOSPHERIC   PRESSURE.        77 

If  a  leak  occurs,  it  is  probably  due  to  uneven  tightening  of  the  joints, 
and  in  most  cases  it  is  better  to  loosen  the  whole  joint  and  shift  it 
a  little  before  tightening  again  rather  than  to  strain  the  screws  that 
are  already  tight,  in  the  hope  of  making  closer  contact. 

142.  From  one  to  three  or  more  hours  after  the  cleaning  operations 
are  completed  and  the  barometer  is  returned  to  its  support  a  series  of 
five  comparative  readings  with  its  companion  or  standard  instrument 
should  be  made. 

143.  Additional  suggestions  for  cleaning  barometers  with  the  Tuch 
cistern. — The  parts  of  the  barometers  with  Tuch  cisterns  can  be 
removed  only  by  the  aid  of  a  special  wrench,  which  will  be  furnished 
when  any  observer  is  authorized  to  clean  such  barometers. 

The  special  points  to  be  observed  are  as  follows: 

Before  fully  screwing  up  and  inverting  the  barometer,  close  tightly 
the  air  vent,  d,  figure  9.  Consult  also  paragraph  17. 

When  about  to  open  the  cistern,  loosen  the  screw,  W,  figure  9,  one 
or  two  turns.  Then  unscrew  the  piece,  q,  being  very  careful  at  this 
time  not  to  unscrew  the  part,  K.  When  completely  unscrewed,  the 
piece,  q,  with  attached  piston,  o,  may  be  wholly  withdrawn,  exposing 
the  mercury.  All  or  nearly  all  of  this  may  be  poured  out  in  the  man- 
ner described  in  paragraph  127  without  danger  of  starting  the  mer- 
cury from  the  end  of  the  barometer  tube.  The  small  portion  of  mer- 
cury that  is  apt  to  remain  should  be  poured  out  afterwards,  removing 
first  the  tube  or  barrel,  K,  then  the  clamp  ring,  ~h,  holding  the  glass 
cistern  in  position  by  the  hand  until  the  last  portion  of  mercury  is 
emptied. 

To  remove  the  barometer  tube  from  the  sheath,  the  cap  of  the  air 
vent  must  be  first  removed. 

It  is  always  best  not  to  disturb  any  more  than  necessary  the  various 
leather  washers  and  fittings. 

144.  In  replacing  the  parts   be  sure  the  cap  of  the  air  vent  is 
screwed  up  before  introducing  the  mercury. 

After  inserting  the  glass  cistern  and  washers  screw  down  upon  it 
the  ring  clamp,  h.  Then  filter  in  as  much  mercury  as  practicable, 
observing  the  precautions  mentioned  hi  paragraph  140.  Next  screw 
down  snugly  the  tube  or  barrel,  K,  and  add  the  remaining  mercury. 
Finally,  replace  the  piston  and  cap,  q. 

If  the  piston  does  not  fit  the  barrel  snugly  enough,  the  washers 

should  be  tightened  by  turning  up  the  screw,  j,  using  the  special 

wrench.     The  parts  of  the  piston  should  not  be  taken  apart  nor  the 

washers  disturbed,  except  possibly  to  tighten  the  screw  mentioned 

i  above. 

During  the  operation  of  replacing  the  piston  it  should  not  close 
down  upon  the  mercury,  which  would  be  subjected  to  severe  pressure 
i  thereby.  At  last,  tighten  the  screw,  Z. 


78        BAROMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC   PRESSURE. 

Before  the  instrument  is  placed  erect  the  piston  should  be  screwed 
up  close  to  the  mercury,  still  leaving  a  small  space,  as  shown  by  the 
air  bubble,  visible  in  the  cistern  when  the  barometer  is  held  nearly 
horizontal. 

145.  Suggestions    about    moving    and    packing    barometers. — Pre- 
paratory to  moving  invert  the  barometer  as  described  in  paragraph 
111. 

The  most  approved  methods  of  packing  barometers  for  transporta- 
tion are  to  be  learned  by  carefully  observing  the  manner  followed  in 
packing  instruments  sent  out  by  this  office.  The  instrument  should 
be  shipped  in  a  horizontal  position.  The  air  -bubble  space  in  the 
cistern  should  be  small,  but  still  sufficient  to  admit  of  expansion 
with  temperature  changes. 

When  carried  about  by  hand,  the  cistern  should  be  uppermost. 

The  barometer  should  be  first  wrapped  in  soft  paper,  then  with  a 
thick  layer  of  cotton  sheeting  and  an  outer  wrapping  of  heavy  paper. 
Thus  prepared,  it  is  then  placed  in  the  middle  of  a  strong  wooden  box 
and  completely  and  closely  surrounded  with  good  excelsior  or  cotton 
or  similar  elastic  packing  material.  The  lid  of  the  box  must  be 
screwed  down,  not  nailed,  and  a  strong  handle  attached  to  the  middle, 
so  that  the  box  may  be  carried  by  one  hand  in  a  horizontal  position. 

146.  Leather  carrying  cases. — In  using  the  leather  carrying  case, 
supplied  when  barometers  are  to  be  transported  by  hand,  secure  the 
barometer  in  the  hinged  wooden  sheath,  being  careful  to  observe  that 
the  latter  closes  tightly  without  straining  either  the  milled  head  for 
regulating  the  vernier  or  the   attached   thermometer.     The  wood 
should  be  neatly  cut  away,  if  necessary,  but  only  sufficiently  to  receive 
these  projecting  parts.     Insert  the  barometer,  cistern  uppermost,  into 
the  leather  case. 

147.  On  steamboats  or  railroads  the  barometer,  if  hung  up  in  any 
manner,  should  be  secured  against  striking  or  pounding  the  side  of 
the  room  or  car.     In  wheeled  vehicles  the  barometer  should  be  car- 
ried by  hand,  supported  by  a  strap  over  the  shoulder,  or  held  upright 
between  the  legs.     It  should  not  be  allowed  to  rest  on  the  floor,  as  a 
severe  jolt  may  break  the  tube.     On  stage  routes,  when  impracti- 
cable to  carry  it  by  hand,  hang  the  barometer  on  a  hook  inside  the 
stage  and  securely  fasten  the  lower  end,  so  that  it  will  not  swing 
when  being  thus  transported.     If  carried  on  horseback,  it  should  be 
strapped  over  the  shoulders  of  the  rider,  where  it  is  not  likely  to  be 
injured. 

148.  Change  of  location. — It  sometimes  becomes  necessary  to  change 
the  location  of  instruments  from  one  office  room  to  another  or  to  a 
different  point  in  the  same  room  and  making  little  or  no  change  of 
elevation.     In  such  cases  the  barometer  box  can  be  moved  bodily 
with  the  instruments  in  place.     The  first  step  is  to  prepare  the  wall 

. 

* 


BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        79 

at  the  new  location  by  setting  up  the  necessary  wood  strips  and  the 
screw  at  the  top  on  which  the  hook  of  the  barometer  box  can  be 
-hung.  If  no  change  is  to  be  made  in  the  elevation  this  screw  must 
be  at  exactly  the  same  height  as  the  corresponding  screw  at  the  old 
location.  After  the  usual  comparative  readings  have  been  made 
and  all  is  prepared  the  barometer  cisterns  will  be  screwed  up  until 
full  of  mercury  and  the  box  moved  bodily  in  an  upright  position  and 
secured  at  the  new  location.  Comparative  readings  should  not  be 
made  after  the  removal  until  an  interval  of  two  or  three  hours  has 
elapsed,  unless  the  temperature  is  practically  the  same  in  the  two 
locations. 

149.  Shipment  of  serviceable  barometers. — Every  possible  care  is 
taken  at  the  central  office  to  secure  the  highest  attainable  precision 
in  the  pressure  observations  at  stations  and  to  ascertain  the  amount  of 
abnormal  errors  that  sometimes  develop  in  the  use  of  instruments. 
To  this  end  defective  instruments  are  called  in  to  the  central  office 
for  recomparison,  and  obviously  it  is  of  the  highest  importance  that 
every  precaution  be  taken  in  packing  such  instruments  to  insure  their 
safe  arrival  at  destination. 

150.  In  reporting  on  defective  or  unserviceable  barometers,  observ- 
ers should  state  clearly  the  condition  of  the  instrument,  and  whether 
or  not  the  mercury  can  be  screwed  up  to  such  an  extent  as  to  permit 
of  forwarding   the  instrument  " mercury  filled."     Such  barometers 
are  called  in  by  the  official  in  charge,  division  of  supplies,  central 
office,  by  express,  and  observers  should  keep  on  hand  constantly  a 
small  supply  of  the  special  I1  Notice,"  "Glass,"  and  "Very  fragile" 
labels,  two  or, three  of  which  should  be  tacked  or  pasted  on  outside 
cover  of  box  in  a  conspicuous  manner. 

151.  Shipment   of  empty  instruments. — When   barometers   are  so 
seriously  defective  that  check  comparisons  can  not  be  made  or  are 
valueless,  observers  will  be  authorized  to  forward  the  barometer  to 
Washington,  first  carefully  emptying  all  the  mercury,  which  will  be 
preserved  in  a  clean  bottle  and  packed  in  the  box  with  the  barometer. 

(B)   CARE    AND    USE    OP   BAROGRAPHS. 

152.  Exposure  of  barographs. — The  general  principles  of  the  expo- 
sure of  barometers  given  in  paragraph  89  apply  to  barographs  also, 
except  that  the  matter  of  light  is  not  so  essential.     Every  precaution, 
however,  must  be  observed  to  prevent  the  instrument  from  being 
exposed  to  great  changes  of  temperature  and  to  direct  influence  of 
sunshine,  etc. 

The    instructions    following    apply    particularly    to    the    Richard 
barograph. 

153.  Adjustment  to  standard  pressure. — When  the  instrument  is  first 
set  up  at  a  place,  the  pen  should  be  made  to  mark,  as  nearly  as  possi- 


80        BAROMETERS   AND  MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

ble,  the  corrected  pressure  (see  paragraph  172)  given  by  a  standard 
mercurial  barometer  at  the  same  place.  This  adjustment  is  made  by 
raising  or  lowering  the  whole  series  of  aneroid  shells  by  means  of  a 
screw  reached  through  a  hole  in  the  base  of  the  instrument  just  under 
the  aneroid  shells.  This  screw  is  turned  by  one  end  of  the  key  sup- 
plied for  winding  the  clockwork.  The  adjustment  of  the  barograph 
to  agree  with  a  standard  barometer  will  rarely  prove  permanent  and 
will  require  a  little  alteration  from  time  to  time,  there  being  a  slight 
tendency  for  an  aneroid  barometer  to  read  too  high  with  age.  It 
is  generally  necessary  to  set  the  barograph  to  standard  at  the  tune  a 
new  sheet  is  put  on,  but  if  the  error  is  small  it  is  better  to  allow  for 
it  than  to  readjust. 

1 54.  Special  adjustment  for  high  elevations. — The  adjustment  afforded 
by  the  screw  underneath  the  base  of  the  instrument  is  not  sufficient 
to  bring  the  pen  to  the  proper  pressure  at  stations  four  thousand  feet 
or  more  above  sea  level.     In  this  case,  and  also  to  prevent  injury  to 
the  barometer  while  in  transit  over  lofty  mountain  passes,  it  is  some- 
times necessary  at  this  office  to  disconnect  the  system  of  levers  from 
the  aneroid  shells  and  to  provide  one  or  more  extra  holes  in  the  stem 
projecting  at  the  top.     Barographs  found  disconnected  in  this  man- 
ner upon  arrival  at  stations  need  simply  that  the  links  be  united  again, 
placing  the  small  pin  in  whichever  hole  will  bring  the  pen  nearest  the 
middle  of  the  record  sheet.     When  shipping  barographs,  if  the  instru- 
ment has  to  pass  over  a  greater  elevation  than  3,000  feet,  the  system 
of  levers  should  be  disconnected,  as  above. 

155.  When  the  pressure  at  any  particular  station  is  such  as  not  to 
be  included  in  the  rulings  on  the  record  sheets  furnished  with  the 
instrument,  observers  will  change  the  numbering  of  the  lines  by 
some  convenient  whole  number  and  adjust  the  pen  of  the  instrument 
accordingly. 

156.  The  sheets  should  be  changed  at  about  noon  on  the  1st,  8th, 
15th,  22d,  and  29th  of  each  month. 

When  a  barograph  is  first  put  in  operation  the  trace  on  the  sheet 
should  start  at  the  proper  date  and  hour,  even  if  near  the  end  of  the 
sheet.  The  roman  numeral,  XII,  at  the  top  of  the  sheet  indicates 
noon,  and  the  letter  "M.,"  midnight. 

If  not  already  done,  the  lower  edge  of  the  sheet  should  be  trimmed 
accurately  parallel  to  the  longitudinal  lines  and  should  rest  closely 
against  the  flange  at  the  bottom  of  the  cylinder  and  the  pen  be 
adjusted  to  the  proper  pressure,  in  accordance  with  instructions  in 
paragraph  153. 

157.  Barograph  docks. — Every  effort  should  be  made  to  regulate 
the  barograph  clock  to  keep  correct  time,  winding  once  a  week,  or 
oftener  if  found  to  give  better  results.     The  instrument  should  be 
inspected  each  day  by  the  observer  in  charge  and  properly  adjusted 


BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        81 

by  liim  when  necessary.  Whenever  a  clock  is  adjusted,  a  marginal 
note  stating  the  fact  should  be  connected  with  the  proper  hour.  If 
the  clockwork  goes  too  fast  or  slow,  it  can  be  regulated  in  the  same 
manner  as  the  movement  of  a  watch,  through  an  opening  in  the 
cylinder  marked  A  (to  " accelerate ")  and  R  (to  "retard"),  correspond- 
ing to  the  letters  "F"  and  "S"  near  the  ends  of  the  regulator  itself. 

158.  Time  error  of  barograph. — The  clock  movements  of  these  baro- 
graphs  often  keep  but  imperfect  time,  and  it  is  important  that  the 
record  be  checked  in  tliis  respect,  so  that  proper  correction  may  be 
made  in  compiling  the  records  of  hourly  readings.     This  result  is 
secured  by  gently  touching  the  lever  of  the  recording  pen,  so  as  to 
deflect  it  and  cause  a  slight  lateral  mark  to  be  made  on  the  record 
sheet  across  the  baromet fie  -trace.     A  mark  of  this  character  will  be 
made  each  day  at  noon,  seventy-fifth  meridian  time;  the  record  sheet 
•will  thus  always  show  how  much  the  barograph  clock  may  be  in  error. 

In  producing  these  marks  great  care  must  be  exercised  not  to  strain 
the  lever  mechanism  in  any  way;  the  weight  of  any  ordinary  lead 
pencil  is  amply  sufficient  to  make  the  mark,  which  should  not  be  more 
than  one-eighth  inch  in  length. 

159.  Pens. — Pens  should  be  kept  clean.     Only  the  standard  register 
ink  should  be  used.     Care  must  be  observed  in  cleaning  a  pen  not  to 
bend  or  deform  the  points  and  render  it  unserviceable.     (See  Circular 
A.  Instrument  Division.) 

160.  Corrections. — Owing  to  imperfections  in  barographs,  more  or 
less  frequent  comparisons  should  be  made  with  standard  instruments, 
and  corrections  applied  according  to  the  scheme  fully  described  in 
Circular  A,  Instrument  Division. 

VI. — CONCERNING    THE    ELEVATION    OF   STATIONS. 

161.  In  the  system  of  the  Weather  Bureau  the  elevation  of  a  station 
is  the  height  above  mean  sea  level  of  the  zero  point — that  is,    the 
"ivory  point" — of  the  barometer  scale,  and  all  measurements  and 
levelings  for  elevations  must  be  made  in  reference  thereto. 

162.  Elevation  determined  by  spirit  level. — Whenever  a  station  is 
established  or  an  office  moved  and  the  elevation  of  the  barometers 
changed,  observers  will  secure  the  services  of  a  competent  surveyor 
or  city  engineer, who  will  run  a  line  of  levels  to  determine   accurately 
the  elevation  of  the  station  above  or  below  the  "plane  of  reference." 
In   many  instances   this  survey  can   be  secured   without  expense 
through  the  courtesy  of  the  Government  or  of  the  city  engineers. 
In  the  remaining  cases  the  cost  will  be  included  with  other  items  of 
expense   incident   to   the   establishment   or  removal   of  the   office, 
authority  for  the  expenditure  being  procured  in  the  usual  manner. 

22937—12 6 


82        BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

All  heights  will  be  given  in  feet  and  hundredths  or  thousandths  of 
a  foot. 

163.  Fixed  point. — The  engineer  will  establish  a  so-called  /'fixed 
point"  or  bench  mark  in  a  permanent  manner  on  the  outer  stone- 
work of  the  building,  from  which  direct  measurements  of  the  height 
of  the  barometers  can  readily  be  made,  or  a  line  of  levels  conven- 
iently run  to  the  " plane  of  reference"  at  any  time. 

164.  Plane   of  reference. — The    " plane    of   reference"    should,   in 
general,  be  the  top  of  rail  at  depot.     In  case,  however,  a  bench  mark 
of  a  permanent  character  and  of  high  precision,  such  as  erecced  by 
the  United  States  Coast  Survey,  the  Lake  Survey,  the  Mississippi 
River  Commission,  or  the  Engineer  Corps,  has  been  established,  this 
will  be  used  as  "plane  of  reference."     The  same  plane  should  be  used, 
as  a  rule,  in  all  measurements  within  the  same  town,  and  will  not  be 
changed  except  for  good  -reason. 

165.  All  levelings  must  be  run  by  a  competent  person  and  the  line 
will  be  run  over  a  closed  circuit  which,  in  the  case  of  a  new  station, 
will  be  from  the  plane  of  reference  through  the  " fixed  point"  to 
the  barometer,  and  back  through  the  "fixed  point"  to  the  plane  of 
reference.     In  the  case  of  the  removal  of  an  office  the  line  will  be 
run  from  the  barometer  in  the  old  office  to  the  barometer  in  the  new 
office,  passing  through  the  "fixed  points"  at  both  the  old  and  the 
new  office,  thence  through  the  "fixed  point"  at  the  new  office  to  the 
"plane  of  reference,"  and  thence  to  the  point  of  starting. 

A  copy  of  the  field  notes,  certified  to  by  the  surveyor  running  the 
levels,  will  be  filed  with  the  report  of  elevation  which  will  be  rendered 
on  Form  No.  1058-Met'l. 

The  observer  will  fully  inform  the  surveyor  concerning  the  fore- 
going provisions  for  running  the  levels. 

166.  When,  for  any  reason,  it  is  impossible  to  run  the  levels  pro- 
vided for  in  paragraphs  162  to  165,  the  observer  will,  in  case  of  the 
change  of  location  of  the  barometers,  or  removal  of  the  office,  make 
a  special  set  of  comparative  barometer  readings;  that  is  to  say,  the 
comparative  barometer  readings  always  required  on  changing  the 
location  of  instruments  will  in  case  levels  can  not  be  run  be  made  in 
three  sets,  as  follows: 

First.  Set  before  removal.  Second.  Set  during  removal;  that  is, 
station  barometer  in  new  office,  extra  barometer  in  old  office.  This 
set  of  readings  will  be  recorded  as  usual  on  Form  No.  1027-Met'l,  and 
will  be  accompanied  by  two  readings  of  the  exposed  thermometer,  to 
be  noted  on  the  margin,  the  first  taken  immediately  before  and  the 
second  immediately  after  the  set  of  barometer  readings.  Third.  Set 
after  removal;  that  is,  both  barometers  in  new  location. 


BAROMETERS    AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        83 

When  there  is  an  assistant  on  station  the  five  readings  of  the  second 
set  must  be  synchronous,  but  in  case  there  is  no  assistant  at  station, 
eight  readings  in  all  will  be  made  alternately  in  the  two  offices,  in  the 
following  order:  No.  1  in  new  office;  Nos.  2  and  3  in  old  office;  Noe, 
4  and  5  in  the  new  office;  Xos.  6  and  7  in  the  old  office,  after  which 
the  extra  barometer  will  be  carried  to  'the  new  office  and  the  eighth 
reading  made  on  the  station  barometer.  Finally,  the  third  set  of 
comparative  readings  wrill  be  made. 

The  interval  between  readings  should  be  as  nearly  uniform  as  pos- 
sible, and  is  left  to  the  convenience  of  the  observer,  depending  upon 
the  distance  between  stations,  etc. 

167.  When  changes  in  the  location  of  the  barometers  that  do  not 
alter  the  elevation  are  authorized,  a  line  of  levels  need  not  be  run, 
and  only  the  usual  comparative  barometer  readings  before  and  after 
removal  will  be  made. 

168.  Reduction  of  observation*  to  a  "station  elevation" — On  January 
1,  1900,  a  specific  elevation  above  sea  level  was  adopted  for  each 
Weather  Bureau  station,  and  for  purposes  of  record  and  publication 
all  barometric  observations  will  be  correlated  to  this  "  adopted  or 
station  elevation."     In  case,  therefore,  an  office  is  moved  to  new 
quarters  and  the  elevation  of  the  barometer  is  thereby  changed,  a 
proper  correction  will  be  applied  to  the  barometric  readings  in  the 
new  location  that  will  reduce  the  observed  readings  to  the  pressure 
appropriate  to  the  " station  elevation,"  notwithstanding  changes  and 
removals.     The  pressure  thus  ascertained  will  be  designated  "  station 
pressure." 

169.  The  " station  elevation"  for  a  station  in  operation  January  1, 
1900,  is  its  elevation  above  sea  level  on  that  date.     For  stations 
closed  before  1900,  or  subsequently  established,  the  elevation  will  be, 
in  general,  the  elevation  above  sea  level  of  the  zero  point  of  the 
barometer  at  the  date  of  closing  or  opening  the  respective  stations. 

170.  Reduction  of  current  observations  to  a  "station  elevation"  in 
accordance  with  the  foregoing  plan  will,  therefore,  be  required  only 
when  changes  are  made  hi  the  elevations  of  the  barometers.     In  all 
such  cases  the  Instrument  Division  of  the  central  office  will  furnish 
a  new  copy  of  the  barometer  correction  card  (Form  No.  1059-Met'l), 
in  which  a  "  removal  correction,"  based  on  the  change  made  in  the 
elevation  of  the  barometers,  will  be  combined  with  the  corrections 
for  local  gravity,  scale  errors,  etc.     The  "sum  of  corrections"  thus 
determined,  together  with  the  "correction  for  temperature,"  will  be 
applied  to  all  recorded  readings  of  barometric  pressure,  and  the  result 
will  be  regarded  as  the  pressure  of  the  air  appropriate  to  the  station 
in  question. 


84        BABOMETEBS    AXD    MEASUEEMENT    OF    ATMOSPHEEIC    PBESSUEE. 

171.  The  following  example  will  elucidate  the  complete  correction 
of  observed  barometer  readings : 

Observed  .barometer  reading  (attached  thermometer,  76.5°) 30.  287 

Correction  for  temperature  (Table  I) —0. 131 

Sum  of  corrections,  Form  No.  1059-Met'l +0.  032 


Total  correction . .  .    -  0.  099 


Station  pressure 30. 188 

The  "total  correction/'  as  shown  above,  will  be  entered  on  Form 
No.  1001-Met'l,  and  applied  to  the  "observed"  reading,  deriving 
thereby  the  pressure  of  the  air  appropriate  to  the  adopted  elevation 
of  the  station;  this  result  will  be  recorded  on  Form  No.  1001-Met'l, 
in  the  columns  for  "station  pressure." 

172.  The  barograph  will  be  adjusted  and  corrected  to  corresponc 
with   the   "station   pressure"    ascertained   as   shown   in   the   above 
example.     See  also  paragraph  153. 

173.  All  pressure   observations   made   at   a  station   and  reduced 
according  to  the  foregoing  plan  are  therefore  strictly  comparable  with 
each  other,  all  being  reduced  to  the  adopted  elevation.     Furthermore, 
a  change  of  elevation  or  removal  of  office  does  not  necessitate  a  new 
table  of  reductions  to  sea  level;  that  is,  all  observations  will  be  reduced 
to  sea  level,  when  required,  by  one  and  the  same  table  of  reduction, 
namely,  that  based  on  the  adopted  elevation  of  the  station. 

174.  Nomenclature. — The  following  nomenclature,  embracing  baro- 
metric terms,  will  be  used,  as  far  as  practicable,  in  the  correspondence, 
records,  and  publications  of  the  Weather  Bureau : 

Actual  elevation. — The  height  of  the  zero  points  of  the  barometers 
of  a  station  above  sea  level. 

Station  elevation. — The  elevation  above  sea  level  adopted  for  a  sta- 
tion as  the  basis  to  which  all  pressure  observations  at  the  station  are 
correlated. 

Observed  reading. — The  direct  result  of  the  reading  of  an  instru- 
ment, uncorrected  for  any  errors. 

Actual  pressure. — Meaning  the  actual  pressure  of  the  air  at  a  barom- 
eter, as  obtained  from  the  observed  reading  after  applying  the  neces- 
sary corrections  for  temperature,  gravity,  and  instrumental  errors. 

Station  pressure. — A  pressure  corresponding  to  an  "adopted  or  sta- 
tion elevation"  which  may  differ  slightly  from  the  actual  elevation  of 
the  barometer.  When  the  "actual"  and  "station"  elevations  are 
the  same  at  any  particular  station,  the  removal  correction  will  be  zero 
and  the  actual  pressure  and  the  station  pressure  are  then  numerically 
equal. 

Reduced  pressure. — The  actual  or  station  pressure  reduced  to  sea 
level,  or  to  some  other  specified  plane. 


BAROMETERS    AND    MEASUREMENT    OF    ATMOSPHERIC    PRKSsrm-:.        85 

Correction  for  scale  errors,  capillarity,  etc. — A  mean  difference  bet  ween 
the  readings  of  a  given  instrument  and  those  of  the  standard  barom- 
eter duly  corrected.  This  quantity  embraces  all  outstanding  error- 
in  the  subdivision  of  the  scale,  or  its  total  length:  errors  in  the  adjust- 
ment of  the  sighting  (»(lge  to  the  zero  line  of  the  vernier:  error-  <>t  cap- 
illarity, imperfect  vacuum,  etc. 

( 'orrection  for  tem/^rature. — The  correction  depending  on  the  tem- 
perature of  the  mercury  and  the  metallic  scale. 
( 'orrection  for  local  gravity: 

(a)  Latitude  term. — The  correction  based  on  the  variation  of  the 

force  of  gravity  with  latitude. 
(6)   Altitude  term. — The    correction    based    on    the    variation    of 

gravity  with  altitude  above  sea  level. 

Removal  correction. — The  correction  necessitated  by  the  removal  of 
an  office,  and  based  on  the  difference  between  the  actual  elevation  of 
the  barometers  in  the  new  location  and  the  adopted  elevation  for  the 
station  in  question.  Strictly  speaking,  the  " removal  correction" 
varies  from  day  to  day,  or  from  winter  to  summer,  with  large  changes 
of  temperature,  but  this  variation,  in  most  cases,  can  be  neglected. 
When,  however,  the  change  in  elevation  is  large,  and  especially  when 
great  differences  of  temperature,  as  between  win'ter  and  summer,  are 
considered,  the  change  in  the  value  of  the  " removal  correction"  may 
exceed  0.01  inch  or  more,  and  it  then  becomes  desirable  to  employ  a 
" variable"  correction,  as  more  fully  explained  in  paragraph  182  (a). 
Sum  of  corrections. — A.  term  embracing  all  the  corrections  that  are 
practically  constant  for.  a  given  instrument  in  a  given  location,  namely, 
the  correction  for  scale  errors,  capillarity  and  gravity,  and  the  removal 
correction.  This  sum  is  given  on  the  certificate  of  corrections  (Form 
No.  1059-Met'l)  furnished  for  each  instrument. 

Total  correction. — The  correction  for  temperature,  plus  the.  "sum  of 
corrections"  as  defined  above. 

Reduction  to  sea  level. — The  quantity  which  must  be  added  to  the 
"actual"  or  "station"  pressure  in  order  to  obtain  the  ''reduced 
pressure. 

Reduction  for  elevation. — A  quantity  which  must  be  added  to  or  sub- 
tracted from  the  pressure  at  a  given  elevation  in  order  to  deduce 
therefrom  the  pressure  appropriate  to  some  other  specified  elevation. 

VII. — SUMMARY    OF    SPECIAL    INSTRUCTIONS    FOR    OBSERVERS    OF    THE 

WEATHER    BUREAU. 

175.  Station  and  extra  barometers. — It  is  the  intention  to  keep  each 
station  supplied  as  far  as  possible  with  two  good  barometers.  The 
names  station  and  extra  apply,  respectively,  to  the  one  used  in  tin- 
regular  observations  and  the  one  held  in  reserve.  Observers  will 
promptly  report,  by  letter,  any  defect  observed  in  either  in-t  ruinent . 
giving  full  details  as  to  its  nature  and  probable  cause 


86        BAROMETERS   AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE. 

176.  Exposure  of  barometers. — Observers  will  expose  their  baro- 
meters and  barographs  in  acordance  with  the  provisions  of  paragraphs 
89  and  152. 

177.  Cisterns  of  extra  barometers. — When  not  in  use  the  cistern  of 
the  extra  barometer  may  be  screwed  up  so  as  to  raise  the  level  of  the 
mercury  about  halfway  up  the  ivory  point.     The  cistern  should  not  be 
filled  nor  the  mercury  forced  to  the  top  of  the  tube/  except  when  the 
barometer  is  to  be  moved. 

178.  Elevation  of  barometers  above  sea  level. — At  the  earliest  practi- 
cable opportunity  after  establishing  a  new  station  or  moving  into  a 
new  office  the  observer  will  ascertain  and  report,  upon  Form  No.  1058- 
Met'l,  the  elevation  of  the  barometer  above  sea  level.     This  measure- 
ment should  refer  to  the  elevation  of  the  lower  end  of  the  ivory 
point  of  the  barometer.     Specific  instructions  in  regard  thereto  are 
given  in  paragraphs  161  to  166,  and  on  the  back  of  the  form  mentioned. 

179.  When  observations  are  commenced  at  a  new  station  and  the 
appropriate  table  of  reductions  to  sea  level  has  not,  as  yet,  been 
received  from  the  central  office,  only  the  corrections  given  on  Form 
No.  1059-Met;l  and  for  temperature  will  be  applied  to  barometer 
readings,  and  the  results  will  be  recorded  on  all  forms  required  and 
telegraphed  in  the  usual  manner  until  tables  of  reduction  are  received. 

180.  Regular  barometric  observations. — Settings  and  readings  of  the 
" station"  barometer,  as  required  by  the  instructions  providing  for 
the  regular  observations  of  the  station,  will  be  made  in  accordance 
with  the  provisions  of  paragraphs  97  to  100,  and  these  readings  will  be 
corrected  for  errors,  as  illustrated  in  the  example  given  in  para- 
graph 171. 

181.  Reduction  of  barometric  readings  to  sea  level. — In  reducing 
barometer  readings  to  sea  level  the  special  tables  supplied  each  station 
according  to  its  elevation  will  be  used. 

The  temperature  argument  will  be  obtained  by  taking  the  mean  of 
the  current  and  the  previous  8  a.  m.  or  8  p.  m.  observation.  For 
example:  Dodge  City,  Kans.,  March  1,  1905,  temperature,  8  a.  m., 
33.7°;  8  p.  m.,  45.4°;  the  mean  of  these,  39.6°,  is  the  temperature  to 
be  used  in  reducing  the  8  p.  m.  reading  to  sea  level: 

182.  Provisional  removal  correction. — When  the  elevation  of  the 
barometer  has  been  changed  by  removal  or  otherwise,  and  the  observer 
has  ascertained  with  reasonable  accuracy  the  new,  that  is,  the  "  actual 
elevation"  (see  paragraph  174),  he  will  immediately  report  the  same 
to  the  central  office,  even  if  Form  No.  1058-Met'l  can  not  be  sub- 
mitted at  the  same  time.     Furthermore,  pending  the  receipt  of  new 
copies  of  Form  No.  10'59-Met'l,  he  will  reduce  his  daily  barometric 
observations  at  the  new  location  by  the  use  of  a  provisional  removal 
correction  deduced  in  the  manner  illustrated  in  paragraph  198.     In  his 


BAROMETERS    AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        87 

letter  the  observer  will  give  the  value  and  algebraic  sign  of  the  cor- 
rection he  proposes  to  use. 

182a.  Variable  removal  correction. — When  extreme  changes  of 
temperature,  such  as  occur  at  different  seasons  of  the  year,  taken  in 
conjunction  with  considerable  changes  of  elevation  due  to  removal 
of  office,  cause  variations  in  the  " removal  correction"  amounting  to, 
or  exceeding  0.010  inch,  it  then  becomes  necessary  to  employ  a 
"  variable  removal  correction,"  which  is  taken  out  for  each  observa- 
tion and  which  depends  upon  the  temperature  of  the  outside  air.  In 
such  cases  this  "variable"  correction  for  different  temperatures  is 
given  in  a  special  table  on  the  back  of  Form  1059-Met'l,  and  is  used 
for  reducing  the  observations  in  the  following  manner: 

The  barometric  observations  will  be  reduced  practically  the  same 
as  heretofore  (see  par.  171),  except  that  the  proper  "sum  of  correc- 
tions" to  be  used  at  each  observation  will  be  taken  from  the  table 
mentioned  above  corresponding  to  the  air  temperature  observed  at 
the  same  time.  It  is  important  to  note  that  this  temperature  is  the 
dry-bulb  reading  in  the  instrument  shelter,  not  that  of  the  attached 
thermometer  of  the  barometer.  The  "Sum  of  corrections"  thus 
taken  from  this  supplemental  table  will  be  added  algebraically  to 
the  temperature  correction  in  the  usual  manner,  and  the  resulting 
total  correction  applied  to  the  barometer  reading,  and  also  entered 
in  the  customary  spaces  on  Form  1001,  in  the  column  headed  "Total 
correction,"  on  pages  2  and  3.  Finally,  in  entering  the  data  on  Form 
1001-Met'l,  a  supplementary  column  will  be  ruled  adjacent  to  the 
date  column  on  the  left.  This  column  will  be  headed:  "Removal 
cor.,"  and  therein  will  be  entered,  from  observation  to  observation, 
the  particular  value  of  the  "Removal  correction"  from  the  table 
given  on  back  of  Form  1059-Met'l,  corresponding  to  the  "Sum  of 
corrections"  employed  in  reducing  the  observations. 

Whenever  new  correction  cards  (Form  1059-Met'l)  are  furnished 
to  a  station  from  the  central  office,  the  cards  previously  in  use  will 
be  destroyed. 

183.  Comparative  barometer  readings. — The  provisions  of  paragraphs 
118  to  121  will  be  observed  in  making  comparative  readings;  the 
results  will  be  tabulated  and  reported  on  Form  No.  1027-Met'l,  and 
the  detailed  instructions  printed  on  the  back  thereof  fully  complied 
with.  Form  No.  1027-Met'l  will  either  be  duplicated  for  the  station 
file,  or  a  letterpress  copy  retained  for  the  station  record. 

Five  comparative  readings  at  intervals  of  hours,  half  hours,  «»r 
quarter  hours  will  be  made  in  accordance  with  the  foregoing  provi- 
sions and  on  the  occasions  specified  as  follow- : 

(a)  When  now  barometers  are  received  for  the  establishment  of  a 
station,  or,  in  general,  whenever  any  serviceable  Weather  Bureau 
barometer  is  received  at  station,  by  transfer  or  otherwise. 


88        BAROMETERS    AND    MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 

(6)  Regular  monthly  readings  will  be  made  on  the  15th  of  each 
month.  When  the  15th  falls  on  a  Sunday  or  a  holiday,  the  readings 
will  be  deferred  until  the  next  regular  working  day. 

(c)  Before  and  after  removal  of  office  or  change  in  the  location  of 
the  barometers,  also  when  a  line  of  levels  is  not  run,  a  set  with  the 
station  barometer  in  new  office  and  extra  in  old  office,  see  para- 
graph 166. 

(d)  Before  and  after  cleaning  a  barometer. 

184.  Except  in  the  case  of  the  regular  monthly  readings,  observers 
will  forward  special  comparative  barometer  readings  with  a  letter  of 
transmittal,  giving  any  pertinent  remarks  relative  to  the  condition 
of  the  instruments,  the  occasion  for  making  the  readings  and  other 
particulars  that  may  enable  the  central  office  to  understand  more 
fully  the  matter  under  consideration. 

185.  Barometers  not  to  be  changed. — Barometers  will  not  be  changed 
in  location  nor  one  instrument  substituted  for  another  without  author- 
ity from  the  central  office,  unless  the  circumstances  are  such  that 
immediate    action   is    necessary.     Neither   the    " station"    nor   the 
" extra"  instrument  will  be  loaned  without  authority. 

186.  Changes  in  the  location  of  barometers,  especially  when  the 
elevation  is  changed,  as  in  the  removal  of  an  office,  should  be  made 
at  the  end  of  the  month,  if  practicable,  and  the  station  barometer 
will  be  moved  in  the  interval  between  the  last  observation  of  one  da}' 
and  the  first  observation  of  the  following  day.     The  foregoing  will 
also  apply,  as  far  as  practicable,  when  a  new  barometer  is  substituted 
for  the  station  instrument. 

187.  Authority  to  dean  barometers. — Authority  to  clean  barometers 
will  be  granted  only  in  special  cases,  and  applications  therefor  should 
be  made  with  due  consideration  of  the  provisions  of  paragraph  116. 
Unless  otherwise  provided  for,  only  one  barometer  will  be  cleaned 
within  any  given  month,  and  then  just  after  the  regular  monthly 
comparative  readings  have  been  made.     The  set  of  readings  required, 
after  cleaning,  may  be  made  the  same  or  the  following  day,  preferably 
the  latter.     The  specific  instructions  for  cleaning  barometers,  para- 
graphs 124  to.  144,  will  be  carefully  observed. 

188.  Requisition  for  mercury. — Mercury  will  be  furnished  to  stations 
only  on  special  requisition,  which  will  be  made  when  the  mercury  is 
needed  for  immediate  use  in  connection  with  the  authorized  cleaning 
and  repairing  of  barometers. 

189.  Impure  mercury. — Mercury  of  an  impure  character  that  has 
been  removed  from  barometers,  or  otherwise  acquired  on  station, 
should  be  carefully  preserved,  and  in  quantities  of  from  a  half  pound 
to  a  pound  or  more  will  be  forwarded  by  mail  to  the  central  office, 
in  strong  bottles  or  other  suitable  receptacles,  securely  wrapped  in 
proper  packing  material  to  prevent  breakage.     The  stopper  of  the 


BAROMET1  IIS     \XD    MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.        89 

bottle  should  fit  tightly  and  be  strongly  tied  and  scaled  it'  practicable. 
The  mercury  is  easily  reduced  to  great  purity  at  this  office  by  distil- 
lation, etc.  The  package  should  Remarked:  Instrument  Division. 

190.  Mercury  not  to  be  removed. — When  an  unserviceable  barometer 
is  returned  to  this  office  the  mercury  will  not  he  removed  except  as 
provided  for  in  paragraph  151,  and  if  not   returned   to  Washington 
with  the  instrument,  will  be  disposed  of  as  specified  in  paragraph  ls«.». 

191.  Barographs. — The  winding  and  regulating  of  barograph  clock-. 
the  changing  of  sheets,  and  the  adjustment  of  pen>  to  standard  pre-- 
sures,  will  be  attended  to  in  accordance  with  the  provisions  of  Section 
(B),  pages  80  and  81. 

1!)2.  Time  error  checked. — The  barograph  will  be  checked  for  time 
error  at  12  noon,  each  day,  in  the  manner  explained  in  paragraph  158. 

193.  Hourly   readings. — Detailed   instructions   for   compiling   and 
transcribing  records  from  barographs  are  given  in  Circular  A,  Instru- 
ment Division. 

194.  Comparison  of  private  barometers. — Observers  will  extend  even 
courtesy  to  shipmasters  and  others  who  may  apply  at  the  office  for 
information  or  submit  barometers  for  comparison,  adjustment,  etc. 
The  mercurial  barometers  at  a  station  can  not  as  a  rule  be  removed 
from  the  office  for  comparison  and  adjustment  of  barometers  on  board 
vessels,  etc.,  but  in  some  cases  aneroid  barometers  serve  very  con- 
veniently for  this  purpose  and  may  be  supplied  when  required. 

A  memorandum  or  tag,  showing  date  of  comparison,  the  correction 
to  be  applied,  and  other  pertinent  facts  should  accompany  an  instru- 
ment that  has  been  compared. 

VIII.— TABLES. 

EXPLANATION    OF  TABLES. 

TABLES  I  AND  II. — Correction  for  ttn,/»r<iinn. 

195.  Tables  I  and  II  give,  in  English  and  metric  measures,  respec- 
tively, the  corrections  that  should  be  applied  to  the  observed  reading> 
of  a  mercurial  barometer  having  a  brass  scale,  in  order  to  eliminate 
from  the  barometer  reading  the  effect  of  temperature  in  expanding 
the  mercury  and  the  scale.     The  following  example  will  elucidate  the 
use  of  either  table: 

The  attached  thermometer  of  a  barometer  re*d« • 

Tin-  l.an.iiH-t.-r  n-a<U 

In  Table  I,  the  pressure  nearest  29.415  is  29.5.  II«.ri/mitally  oppo- 
site 7'J..r)°  in  the  vertical  column  under  this  pre^ure  \\e  find  0.117. 
which  is  the  correction  required,  and  we  note  that  it  must  be  sub- 
tracted. 


90        BABOMETERS   AND  MEASUREMENT    OF   ATMOSPHERIC    PRESSURE. 

The  reading  corrected  for  temperature  is,  therefore  ....................  .....  29.  415 

—.117 

29.  298 

196.  Interpolation    explained.  —  Suppose    the    pressure    had    been 
29.281,  which  is  about  midway  between  29  and  29.5.     The  correc- 
tion in  this  case  should  be  about  midway  between  0.115  and  0.117, 
or  0.116.     The  same  rules  must  be  followed  if  the  temperature  is 
intermediate  between  the  values  of  the  table. 

TABLES  III  AND  IV.  —  Influence  of  gravity  on  barometric  observations. 

197.  Table  III  gives  the  correction  required  to  reduce  readings  of  - 
the  mercurial  barometer  to  standard  gravity  at  latitude  45°. 

There  is,  in  addition  to  the  above,  another  gravity  correction 
required  to  eliminate  the  effect  of  change  of  gravity  with  elevation. 
This  is  small  and  is  given  in  Table  IV. 

The  aneroid  barometer  does  not  require  any  gravity  correction. 

Example:  For  latitude  38°  and  pressure  29  inches,  the  gravity 
correction  is  minus  0.018  of  an  inch.  For  an  altitude  of  2,490  feet  and 
a  pressure  of  27.3  inches  the  gravity  correction  is  —0.004  of  an  inch. 

TABLE  V.  —  Pressure  in  inches  corresponding  to  changes  of  100  feet  in  elevation. 

198.  Table  V  is  employed  for  computing  "removal  corrections," 
paragraph  182,  and  its  use  is  illustrated  by  the  following  example: 

Suppose  the  "  adopted  or  station  elevation"  of  a  given  station  is 
1,482.7  feet,  and  that  after  removal  the  "actual  elevation"  is  1,516.4 
feet;  hence  the  change  in  elevation  will  be  33.7  feet. 

Suppose,  also,  that  the  mean  annual  temperature  at  the  station  is 
56°. 

Table  V  gives  Od03  for  1,500  feet  and  temperature  56°.  To  reduce, 
therefore,  from  the  "actual  elevation"  of  1,516.4  feet  to  the  "station 
elevation"  of  1,482.7  feet,  the  following  correction  is  necessary,  viz, 

oo  7 

0.103  =  0.035,  which  is  the  "removal  correction"  required  on 


Form  No.  1059-Met'l.     The  correction  in  this  case  must  be  used  with 
the  plus  sign. 

TABLES  VI  AND  VII.  —  Determination  of  heights  by  barometer. 

199.  The  use  of  these  tables  requires  that  at  least  two  observations 
of  the  temperature  and  the  pressure  of  the  air  be  made,  simulta- 
neously if  possible,  at  two  stations.  The  elevation  of  one  of  the 
stations  must  be  known. 

From  Table  VI  we  find  the  first  approximate  difference  of  elevation 
of  the  two  stations.  Table  VII  gives  the  allowance  that  must  be  made 
on  account  of  the  temperature  of  the  air. 


BAROMETERS   AND   MEASU  IM..M  KNT    OF    AT.MusPHERIC    PRESSURE.        91 

To  be  strictly  accurate,  allowance  should  also  be  made  for  three 
other  effects:  (a)  For  the  amount  of  moisture  in  the  air;  (6)  for 
the  effect  of  gravity  on  the  weight  of  the  air:  (c)  for  the  effect  of 
gravity  on  the  weight  of  the  mercury  (not  required  when  the  aneroid 
is  used  or  when  the  readings  of  a  mercurial  barometer  are  separately 
corrected  for  gravity).  By  neglecting  these  effects  in  computing  a 
high  elevation,  say  10,000  feet,  other  conditions  being  average,  an 
error  of  fully  100  feet  may  be  made.  Greater  inaccuracies  than  this, 
however,  are  likely  to  result  if  the  computations  are  based  upon  only 
a  few  barometric  and  temperature  readings, -and  especially  if  the 
readings  are  not  strictly  simultaneous.  Moreover,  the  air  tempera- 
ture required  is  the  mean  temperature  of  the  whole  column  between 
the  two  stations,  but  this  is  only  approximately  given  by  the  mean 
of  the  observed  temperatures. 

The  two  tables  following  are,  therefore,  sufficiently  accurate  for 
use  of  tourists  who  desire  a  knowledge  of  the  approximate  altitude 
corresponding  to  the  more  or  less  limited  and  incomplete  observations 
they  may  make.  Example: 

Barometer  reading  at  base  station inches. .  27. 58 

Barometer  reading  at  upper  station do 21. 47 

Air  temperature  at  base  station degrees. .        68 

Air  temperature  at  upper  station do 42 

Estimated  mean  temperature  of  air  column  is— 

68  +  42     110     ~o_T 
.  2         °   2    ~55    -1' 

fa  Feet. 

Table  W,  at  21 .47,  gives 9»  002 

at  27.58,  gives 2,195 

First  approximate  difference  of  elevation 6, 807 

Tablet  for  T  =  55°  and  6,800  feet,  gives: 

JUL  Feot 

Allowance  for  temperature  of  air 

Hence  difference  of  elevation  is 

Suppose  the  elevation  of  the  base  station  is *»  ^ 

Then  the  elevation  of  upper  station  is 


92        BAROMETERS    AND    MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 
TABLE  I. — Correction  of  mercurial  barometer  for  temperature,  English  measures. 


Observed  reading  of  the  barometer,  in  inches. 

°F. 

17 

18 

19 

20 

21 

22 

23 

24         25         26         27         28         29 

30 

31 

Inches.                                   ADD.                                  Inches. 

-20 

0.075 

0.080  '0.084 

0.089   0.093 

0.  098  JO.  102 

0.107 

0.111 

0.115 

0.120 

0.124 

0.129 

0.133 

0.138 

-19 

.074 

.078     .083 

.087     .091 

.  096     .  100 

.104 

.109 

.113 

.117 

.122 

.126 

.130 

.135 

-18 

.072 

.077 

.081 

.085     .089 

.094  i  .098 

.102 

.106 

.111 

.115 

.119 

.123 

.128 

.132 

-17 

.071 

.075 

.079 

.083     .087 

.  .  092     .096 

.100 

.104 

.108 

.112 

.117 

.121 

.125 

.129 

-16 

.069 

.073 

.077 

.  081      .  086 

.090 

.094 

.098 

.102 

.106 

.110 

.114 

.118 

.122 

.126 

-15 

.068 

.072 

.076 

.080     .084 

.088 

.092 

.096 

.100 

.103 

.107 

.111 

.115 

.119 

.123 

-14 

.066 

.070 

.074 

.  078     .  082 

.086 

.089 

.093 

.097 

.101 

.  105      .  109 

.113 

.117 

.121 

-13 

.065 

.068 

.072 

.076     .080 

.084 

.087 

.091 

.095 

.099 

.  103      .  106 

.110 

.114 

.118 

-12 

.063 

.067 

.070 

.  074     .  078 

.082 

.085 

.089 

.093 

.096 

.  100     .  104 

.107 

.111 

.115 

-11 

.061 

.065 

.069 

.072  |  .076 

.080 

.083 

.087 

.090 

.094 

.  098     .  101 

.105 

.108 

.112 

-10 

.060 

.063 

.067 

.070     .074 

.077 

.081 

.085 

.088 

.092 

.095 

.099 

.102 

.106 

.109 

-  9 

.058 

.062  !  .065 

.069 

.072 

.075 

.079 

.082 

.086 

.089 

.093 

.096 

.•099 

.103 

.106 

-  8 

.057 

.060     .063 

.  067     .  070 

.073 

.077 

.080 

.083 

.087 

.090 

.094 

.097 

.100 

.104 

-  7 

.055 

.058    -.062 

.065  .  .068 

.071 

.075 

.078 

.081 

.084 

.088 

.091 

.094 

.097 

.101 

-  6 

.054 

.057     .060 

.063 

.066 

.069 

.073 

.076 

.079 

.082 

.  085      .  088 

.092 

.095 

.098 

-  5 

.052 

.055 

.058 

.061 

.064 

.067 

.070 

.074 

.077 

.080 

.083 

.086 

.089 

.092 

.095 

-  4 

.051 

.054     .056 

.059 

.062 

.065     .068 

.071 

.074 

.077 

.080 

.083 

.086 

.089 

.092 

-  3 

.049 

.052 

.055 

.058 

.061 

.063      .066 

.069 

.072 

.075 

.078 

.081 

.084 

.086 

.089 

-  2 

.047 

.050 

.053 

.056 

.059 

.061      .064 

.067 

.070 

.073 

.075 

.078 

.081 

.084 

.087 

-  1 

.046 

.  049     .  051 

.054     .057 

.059  1  .062 

.065 

.067 

.070 

.073 

.076 

.078 

.081 

.084 

0 

.044 

.047 

.050 

.052     .055 

.057     .060 

.063 

.065 

.068 

.070 

.073 

.076 

.078 

.081 

1 

.043 

.045 

.048 

.050 

.053 

.055 

.058 

.060 

.063 

.065 

.068 

.070 

.073 

.076 

.078 

2 

.041 

.044 

.046 

.049 

.051 

.053 

.056 

.058 

.061 

.063 

.065 

.068 

.070 

.073 

.075 

3 

.040 

.042 

.044 

.047 

.049 

.051 

.054 

.056 

.058 

.061 

.063 

.065 

.068 

.070 

.072 

4 

.038 

.040 

.043 

.045 

.047 

.049 

.052 

.054 

.056 

.058 

.061 

.063 

.065 

.067 

.070 

5 

.037 

.039 

.041 

.043 

.045 

.047 

.049 

.052 

.054 

.056 

.058 

.060 

.062 

.065 

.067 

6 

.035 

.037 

.039 

.041 

.043 

.045 

.047 

.049 

.052 

.054 

.056 

.058 

.060 

.062 

.064 

7 

.033 

.035 

.037 

.039 

.041 

.043 

.045 

.047 

.049 

.051 

.053 

.055 

.057 

.059 

.061 

8 

.032 

.034 

.036 

.038 

.039 

.041 

.043 

.045 

.047  !  .049 

.051 

.053 

.054     .056 

.058 

9 

.030 

.032 

.034 

.036 

.038 

.039 

.041 

.043 

.045      .  046 

.048 

.050 

.052     .054 

.055 

10 

.029 

.031 

.032 

.034 

.036 

.037 

.039    j  .041 

.042     .044 

.046 

.047 

.049     .051 

.053 

11 

.027 

.029 

.030 

.032 

.034 

.035 

.037 

.039 

.040     .042 

.043 

.045 

.047      .048 

.050 

12 

.026 

.027 

.029 

.030 

.032 

.033 

.035 

.036 

.038 

.039 

.041 

.042 

.044     .045 

.047 

13 

.024 

.026 

.027 

.028 

.030 

.031 

.  033      .  034 

'.036 

.037 

.038 

.040 

.041      .043 

.044 

14 

.023 

.024 

.025 

.027 

.028 

.029 

.031    1  .032 

.033 

.035 

.036 

.037 

.039     .040 

.041 

15 

.021 

.022 

.024 

.025 

.026 

.027 

.029 

.030 

.031 

.032 

.033 

.035 

.036     .037 

.038 

16 

.020 

.021 

.022 

.023 

.024 

.025 

.026 

.028 

.029      .030 

.031 

.032 

.033      .034 

.036 

17 

.018 

.019 

.020 

.021 

.022 

.023 

.024    1  .025 

.  026     .  027 

.029 

.030 

.  031      .  032 

.033 

18 

.016 

.017 

.018 

.019 

.020 

.021 

.022    !  .023 

.024 

.025 

.026 

.027 

.  028     .  029 

.030 

19 

.015 

.016 

.017 

.018 

.018 

.019 

.020 

.021 

.022 

.023 

.024 

.025 

.025      .026 

.027 

20 

.013 

.014 

.015 

.016 

.016 

.017 

.018 

i  .019 

.  020     .  020 

.021 

.022 

.  023      .  024 

.024 

21 

.012 

.012 

.013 

.014 

.015 

.015 

.016 

.017 

.017 

.018 

.019 

.019 

.020     .021 

.022 

22 

.010 

.011 

.011 

.012 

.013 

.013 

.014 

.014 

.015 

.016 

.016 

.017 

.017     .018 

.019 

23 

.009 

.009 

.010 

.010 

.011 

.011 

.012 

.012 

.013     .013 

.014 

.014 

.015      .015 

.016 

24 

.007 

.008 

.008 

.008 

.009 

.009 

.010 

.010 

.011  !  .011 

.011 

..012 

.012     .013 

.013 

25 

.006 

.006 

.006 

.007 

.007 

.007 

.008 

.008 

.008      .009 

.009 

.009 

.  010  |  .  010 

.010 

26 

.004 

.004 

.005 

.005 

.005 

.005 

.005 

.006 

.006     .006 

.006 

.007 

.007  !  .007 

.007 

27 

.003 

.003 

.003 

.003 

.003 

.003 

.003 

!  .004 

.004     .004 

.004 

.004 

.004      .004 

.005 

28 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

.001 

.002 

.002 

.002     .002 

.002 

j 

1 

1 

SUBTRACT. 

29 

0.001    0.001 

0.001   0.001   0.001  iO.OOl   0.001 

0.001 

0.001    0.001   0.001    0.001 

0.001 

0.001 

0.001 

30 

.  002     .  002 

.002  i  .002     .003 

.003     .003 

.003 

.003  ,  .003     .003     .003 

.004     .004 

.004 

31 

.  004     .  004 

.004     .004     .005 

.005  '  .005 

.005 

.Oa5      .006  i  .006     .006 

.006  1  .006 

.007 

32 

.005      .006 

.006  !  .006     .006 

.007  !  .007 

.007 

.008     .008     .008     .009 

.009     .009       .009 

33 

.007     .007 

.008  ;  .008      .008 

.009  i  .009 

.010 

.010     .010     .011      .011 

.012     .012 

.012 

34 

.008     .009 

.009     .010     .010 

.011      .011 

.012 

.012 

.013     .013      .014 

.014 

.015 

.015 

35 

.  010     .  010 

.011  j  .012     .012 

.013      .013 

.014 

.014 

.015      .016     .016 

.017 

.017 

.018 

36 

.011      .012 

.013     .013     .014 

.  015      .  015 

.016 

.017 

.017     .018     .019 

.019     .020       .021 

37 

.013     .014 

.014     .015      .016 

.017     .017 

.018 

.019 

.  020     .  021      .  021 

.  022      .  023       .  024 

38 

.014     .015 

.016  :  .017     .018 

.  019     .  020 

.020 

.021 

.  022     .  023      .  024 

.025      .026       .026 

39 

.016     .017 

.018     .019     .020 

.021  ;  .022 

.023 

.024 

.024  !  .025      .026 

.  027      .  028       .  029 

40 

.018     .019 

.020     .021  !  .022 

.  023     .  024 

.025 

.026 

.027  j  .028     .029 

.  030     .  031        .  032 

| 

i 

BAROMKTKRS    .\M»    .M  KAST  H  K M  KNT    oi     ATMOSPHERIC    PRESSURE.        93 
TABLE  I. — Correction  of  mercurial  barometer  for  temperature,  English  measures Con. 


°F. 

Observed  n-ii.Jinjr  of  the  barometer,  in  in. 

17        17.5       18 

lx  :. 

19 

19.5       20 

20.5       21 

.'I.:, 

22 

22.5 

23 

23.5 

24 

Inches. 

SUBtBACT. 

*     IncKtt. 

35 
35.5 

0.010  o.nio 
.011      .oil 

t.010   0.011    0.011 
.011  I  .012     .012 

0.011 
.012 

1.012 

.012 

0.012 

.ou 

0.012 
.013 

0.012 
.013 

0.013 
.014 

0.013 
.014 

0.013  0.014 
.014     .015 

0.014 
015 

36 
36.5 

.011       .OIL' 

.012     .012 

.013 

.013 

.013 

.013 

.013 
.014 

.013 
.014 

.011 

.014 

.014 
.015 

.014 
.015 

.014 
.015 

.015 
.016 

.015 
.016 

.015     .016 
.  olt,     .017 

.016 
017 

:<7 

.013     .013 

.014 

.014 

.014 

.015 

.015 

.016 

.016 

.016 

.017 

.017 

.017     .018 

.018 

37.5 

.014     .014 

.014 

.015 

.015 

.016 

.016 

.017 

.017 

.017 

.018 

.018 

.  019     .  019 

.019 

38 

.  014     .  015 

.015 

.016 

.016 

.017 

.017 

.017 

.018 

.016 

.019 

.019 

.020     .020 

.020 

38.5 

.015     .016 

.016 

.016 

.017 

.017 

.018 

.018 

.019 

.019 

.020 

.020 

.021     .021 

.021 

39           .016     .OH, 

.017 

.017 

.018 

.018 

.019 

.019 

.020 

.020 

.021 

.021 

.022     .022 

.023 

.<'.».  :,      .  017 

.017 

.018 

.018 

.019 

.019 

.080 

.020 

.021 

.021 

.022 

.022 

.023     .023 

.024 

40      1     .018      .(Ms 

.019     .019 

.020 

.020 

.021 

.021 

.022 

.022 

.023 

.023 

.024 

.024 

.025 

40.5        .01*      .01!) 

.019     .020 

.020 

.021 

.022 

.022 

.023 

.023 

.024 

.024 

.025 

.026 

41  /       .019      .(tin 

.020 

.021 

.021 

.022 

.022 

.023 

.024 

.024 

.006 

.025 

.trji,      .IL',, 

.027 

4175 

.020     .020 

.021 

.022 

.022 

.023 

.023 

.024 

.025 

.025 

.026 

.026 

.027 

.027 

UN 

42 

.021      .021 

.022 

.022 

.023 

.024 

.024 

.025 

.025 

.026 

.027 

.027 

.028     .029 

.029 

42.5       .021      .022 

.023 

.023 

.024     .025 

.025 

.026 

.026 

.027 

.028     .028 

.029 

.030 

.030 

43           .022 

.023 

.023 

.024 

.025     .025 

.026 

.027 

.027 

.028 

.029  I  .029 

.030 

.031 

.031 

43.  o        .  023 

.024 

.024 

.025 

.02ii      .026 

.027 

.028 

.028 

.029 

.030     .030 

.031 

.032 

.032 

44           .024 

.024 

.025 

.026 

.026      .027 

.028 

.029 

.029 

.030 

.031 

.031 

.032 

.033 

.033 

44.  5        .  024 

.025 

.026 

.027 

.027      .028 

.088 

.030 

.030 

.031 

.032 

.032 

.033 

.034 

.035 

45           .025 

.026 

.027 

.027 

.028  !  .029 

.030 

.030 

.031 

.032 

.033 

.033 

.034 

.035 

.036 

45.5       .026 

.027 

.028 

.028 

.029     .030 

.031 

.031 

.032 

.033 

.034 

.034 

.035 

.036       .037 

4»J           .027 

.028 

.028 

.029 

.030 

.031 

.031 

.032 

.033 

.034 

.035 

.035 

.036 

.037       .038 

4.,.:, 

.028 

.028 

.029 

.030 

.031 

.032 

.032 

.033 

.034 

.035 

.036 

.036 

.037 

.i«x 

.039 

47 

.028 

.029 

.030 

.031 

.032 

.032 

.033 

.034 

.035 

.036 

.037 

.037 

.038 

.039 

.040 

- 

47.5       .029 

.030 

.031 

.032 

.(Ktt      .033 

.034 

.035 

.036 

.037 

.038 

.038 

.039 

.040 

.041 

.030 

.031 

.032 

.032 

.033      .034 

.035 

.036 

.037 

.038 

.039 

.040 

.040 

.041 

.042 

48.  5       .031 

.032 

.032 

.033 

.(«4      .035 

.036 

.037 

.038 

.039 

.040 

.041 

.041 

.042 

.043 

49           .031 

.032 

.033 

.034 

.  035      .  036 

.037 

.038 

.039 

.040 

.041 

.042 

.042 

.043 

.044 

49.5       .032 

.033 

.034 

.035 

.i«6     .037 

.038 

.039 

.040 

.041 

.042 

.043 

.044 

.044 

.045 

.033 

.034 

.035 

.036 

.037 

.038 

.039 

.040 

.041 

.042 

.643 

.044 

.045 

.046 

.046 

50.  5       .034 

.035 

.036 

.037 

.038 

.039 

.040 

.041 

.042 

.043 

.044 

.045 

.046 

.047 

.048 

51           .034 

.035 

.036 

.038 

.039 

.040 

.041 

.042 

.043 

.044 

.045 

.046 

.047 

.048 

.049 

51.5       .035 

.036 

.037 

.038 

.039 

.040 

.041 

.042 

.044 

.045 

.046 

.047 

.048 

.049 

.050 

52           .036 

.037 

.038 

.039 

.040 

.041 

.042 

.043 

.044 

.046 

.047 

.048 

.049 

.050 

.051 

52.5       .037 

.038 

.039 

.040 

.041      .042 

.043 

.044 

.045 

.047 

.04* 

.049 

.050 

.051 

.052 

53-        .038 

.039 

.040 

.041 

.042     .043 

.044 

.045 

.046 

.047 

.049 

.050 

.051 

.052 

.053 

53.5       .038 

.039 

.041 

.042 

.043     .044 

.045 

.046 

.047 

.048 

.050 

.051 

.052 

.053 

.054 

54           .039 

.040 

.041 

.043 

.044      .045 

.046 

.047 

.048 

.049 

.051 

.052 

.053 

.054 

.055 

54.5       .040 

.041 

.042 

.043 

.045     .046 

.047 

.048 

.049 

.050 

.052     .053 

.054 

.055 

.056 

.55           .041 

.042 

.043 

.044 

.045     .047 

.048 

.049 

.050 

.051 

.053 

.054 

.055 

.056 

.057 

55.5       .041 

.043 

.044 

.045 

.046  i  .047 

.049 

.050 

.051 

.052 

.054 

.055 

.056 

.057 

,068 

56 

.042 

.043 

.045 

.046 

.047     .048 

.050 

.051 

.052 

.053 

.055 

.056 

.057 

.058 

.060 

56.5 

.043 

.044 

.045 

.047 

.048     .049 

.050 

.052 

.053 

.054 

.056 

.057 

.058 

.059 

.081 

57 

.044 

.045 

.046 

.048 

.049     .050 

.051 

.053 

.054 

.055 

.057 

.058 

.059 

,008 

.062 

57.5 

.044 

.046 

.047 

.048 

.050     .051 

.052 

.054 

.055 

.056 

.058 

.059 

.OJOI 

.061 

.063 

58 

.045 

.047 

.048 

.049 

.051 

.052 

.053 

.055 

.056 

.057 

.aw 

.060 

.061 

.(m       .064 

.046 

.048 

.049 

.050 

.051      .053 

.054 

.055 

.057 

.058 

.060 

.061 

.062 

.(M       .065 

59 

.047 

.048 

.050 

.051 

.052     .054 

.055 

.056 

.058 

.059 

.061 

.062 

.063 

.(H,:, 

,606 

59.5 

.048 

.049 

.050 

.052 

.053     .055 

.056 

.057 

.059 

.060 

.001 

.083 

.084 

.«60 

.087 

60 

.048, 

.050      .1151 

.053 

.054     .055 

.057 

.058 

.060 

.061 

.062 

.064 

.065 

.087 

.068 

60.5 

..14'. 

.050  !  .052 

.053 

.055     .056 

.058 

.059 

.061 

.062 

.063 

.065 

.066 

.068 

,008 

til 

.050 

.051      .053 

.054 

.056     .057 

.059 

.060 

.062 

.063 

.OM 

.066 

.067 

,000 

.070 

',!..-, 

.051 

.052 

.054 

.055 

.057  ;  .058 

.060 

.061 

.062 

.004 

.065 

.067 

.068 

.070 

.071 

62 

.051 

.053 

.054 

.056 

.057     .059 

.060 

.008 

.063 

.065 

.066 

.068 

.069 

.071 

.073 

62.5 

.052 

.054 

.055 

.057 

.058 

.060 

.061 

.063 

.064 

.066 

.067 

.069 

.071 

.072 

.074 

63 

.053 

.OM 

.  o5«; 

.058 

.059 

.061 

.062 

.064 

.065 

.067 

,068 

.070 

.072 

.073 

.075 

63.5 

.054 

.055 

.057 

.058 

.060 

.062 

.063 

.065 

.066 

.068 

,068 

.071 

.073 

.074 

.076 

t.4 

.054 

.056 

.on 

.059 

.061 

.062 

.064 

.006 

.067 

.069 

.070 

.072 

.074 

.075 

.077 

64.5 

.055 

.057 

.068 

.060 

.062 

.063 

.065 

.067 

.068 

.070 

.071 

.073 

.075 

.076 

.078 

10 

.056 

.058 

.059 

.061 

.063 

.064 

.066 

.067 

.069 

.071 

.072 

.074 

.076 

.077 

.079 

1)5.5 

.057 

.058 

.060 

.062 

.063 

.065 

.067 

.068 

.070 

.072 

.073 

.075 

.077 

.078 

.060 

66 

.057 

.059 

.061 

.063 

.064 

.008 

.068 

.068 

.071 

.073 

.074 

.076 

,ora 

.079 

.081 

66.5 

.058 

.060 

,008 

.001 

.065     .067 

.069 

.070 

.072 

.074 

.075 

.077 

.079 

.081 

.082 

1,7 

.059 

..061 

.062 

.064 

.066     .068 

.069 

.071 

.073 

.075 

.076 

.078 

,080 

,088 

.MX;.; 

94        BAROMETERS    AND    MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.  ' 
TABLE  I.-— Correction  of  mercurial  barometer  for  temperature,  English  measures— Con. 


Observed  reading  of  the  barometer,  in  inches. 

°F. 

24 

24.  5  I     25       25.5  |    26 

26.5 

27 

27.5 

28 

28.5 

29 

29.  5       30 

30.5 

31 

Inches.                 .          SUBTRACT.                                   Inches. 

35 

0.014 

0.014 

0.014 

0.015   0.015   0.015 

0.016 

0.016 

0.016 

0.016 

0.017 

0.017 

0.017   0.018     0.018 

35.5 

.015 

.015 

.016 

.016     .016     .017 

.017 

.017 

.017 

.018 

.018 

.018 

.019  i 

.019       .019 

36 

.016 

.016 

.017 

.017     .017      .018 

.018 

.018 

.019 

.019 

.019 

.020 

.020 

.  020       .  021 

36.5 

.017 

.017 

.018 

.018  '  .019  ;  .019 

.019 

.020 

.020 

.020 

.021 

.021 

.021  i 

.  022       .  022 

37 

.018 

.019 

.019 

.019  ;  .020 

.020 

.021 

.021 

.021 

.022 

.022 

.022 

.023 

.  023       .  024  . 

37.5 

.019 

.020 

.020 

.021  ;  .021 

.021 

.022 

.022 

.023 

.023 

.023 

.024 

.024 

.025       .025 

38 

.020 

.021 

.021 

.022  i  .022 

.023 

.023 

.023 

.024 

.024 

.025 

.025 

.026 

.026  :     .026 

38.5 

.021 

.022 

.022 

.023  ;  .023  i  .024 

.024 

.025 

.025 

.026 

.026 

.026 

.027 

.027  1     .028 

39 

.023 

.023 

.024 

.  024  J  .  024  |  .  025 

.025 

.026 

.026 

.027 

.027 

.028 

.028 

.029       .029 

-39.5 

.024 

.024 

.025 

.025  i  .026 

.026 

.027 

.027 

.028 

.028 

.029 

.029 

.030 

.030       .031 

40 

,  .025 

.025 

.026 

.026     .027 

.027 

.028 

.028 

.029 

.029 

.030 

.030 

.031 

.031       .032 

40.5 

.026 

.026 

.027 

.027     .028     .029 

.029 

.030 

.030 

.031 

.031 

.032 

.032 

.033       .033 

41 

.027 

.027 

.028 

.029     .029      .030 

.030 

.031 

.031 

.032 

.033 

.033 

.034 

.034       .035 

41.5 

.028 

.029 

.029 

.030     .030  !  .031 

.032 

.032 

.033 

.033 

.034 

.034- 

.035 

.  036       .  036 

42 

.029 

.030 

.030 

.031     .032 

.032 

.033 

.033 

.034 

.035 

.035 

.036 

.036 

.037,     .038 

42.5 

.030 

.031 

.031 

.032 

.  033      .  033 

.034 

.035 

.035 

.036 

.036 

.037 

.038 

.038 

.039 

43 

.031 

.032 

.033 

.033 

.034 

.035 

.035 

.036 

.0?6 

.037 

.038 

.038 

.039 

.040 

.040 

43.5 

.032 

.033 

.034 

.034  i  .035 

.036 

.036 

.037 

.038 

.038 

.039 

.040 

.040 

.041 

.042 

44 

.033 

.034 

.035 

.036     .036 

.037 

.038 

.038 

.039 

.040 

.040 

.041 

.042 

.042. 

.043 

44.5 

.035 

.035 

.036 

.037     .037 

.038 

.039 

.040 

.040 

.041 

.042 

.042 

.043 

.044 

.045 

45 

.036 

.036 

.037 

.038 

.039 

.039 

.040 

.041 

.042 

.042 

.043 

.044 

.045 

.045 

.046 

45.5 

.037 

.037 

.038 

.039 

.040 

.041 

.041 

.042 

.043 

.044 

.044 

.045 

.046 

.047 

.047 

46 

.038 

.039 

.039 

.040 

.041 

.042 

.043 

.043 

.044 

.045 

.046 

.046 

.047 

.048 

.049 

46.5 

.039 

.040 

.041 

.041 

.042 

.043 

.044 

.045 

.045 

.046 

.047 

.048 

.049 

.049 

.050 

47 

.040 

.041 

.042 

.042 

.043 

.044 

.045 

.046 

.047 

.047 

.048 

.049 

.050 

.051 

.052 

47.5 

.041 

.042 

.043 

.044 

.045 

.045 

.046 

.047 

.048 

.049 

.050 

.050 

.051 

.052 

.053 

48 

.042 

.043 

.044 

.045 

.046 

.047 

.047 

.048 

.049 

.050 

.051 

.052 

.053 

.054 

.054 

48.5 

.043 

.044 

.045 

.  046     .  047 

.048 

.049 

.050 

.050 

.051 

.052 

.053 

.054 

.055 

.056 

49 

.044 

.045 

.046 

.047  !  .048 

.049 

.050 

.051 

.052 

.053 

.054 

.054 

.055 

.056 

.057 

49.5 

.045 

.046 

.047 

.048 

.049 

.050 

.051 

.052 

.053 

.054 

.055 

.056 

.057 

.058 

.059 

-50 

.046 

.047 

.048 

.049     .050 

.051 

.052 

.053 

.054 

.055 

.056 

.057 

058 

.059 

.060 

50.5 

.048 

.049 

.050 

.051     .052 

.053 

.054 

.055 

.055 

.056 

.057 

.058 

!  059 

.060 

.061' 

51 

.049 

.050 

.051 

.052     .053 

.054 

.055 

.056 

.057 

.058 

.059 

.060 

.061 

.062 

.063 

51.5 

.050 

.051 

.052 

.053     .054     .055 

.056 

.057 

.058 

.059 

.060 

.061 

.062 

.063 

.064 

52 

.051 

.052 

.053 

.054     .055     .056 

.057 

.058 

.059 

.060 

.061 

.062 

.064 

.065 

.066 

52.5 

.052 

.053 

.054 

.055 

.056 

.057 

.058 

.059 

.061 

.062 

.063 

.064 

.065 

.066 

.067 

—  53 

.053 

.054 

.055 

.056 

.057 

.059 

.060 

r66t- 

-^962. 

.063 

.064 

.066 

.067 

.068 

53.5 

.054 

.055 

.056 

.057     .059 

.060 

.061 

.062 

.063 

.065 

*066 

.068 

.069 

.070 

54 

.055 

.056 

.057 

.059 

.060 

.061 

.062 

.063 

.064 

!066 

.067 

.068 

.069 

.070 

.071 

54.5 

.056 

.057 

.059 

.060 

.061 

.062 

.063 

.064 

.066 

.067 

.068 

.069 

.070 

.071 

.073 

55 

.057 

.059 

.060 

.061 

.062 

.063 

.064 

.066 

.067 

.068 

.069 

.070 

.072 

.073 

.074 

55.5 

.058 

.060 

.061 

.062 

.063     .065 

.066 

.067 

.068 

.069 

.071 

.072 

.073 

.074 

.075 

56 

.060 

.061 

.062 

.063 

.064 

.066 

.067 

.068 

.069 

.071 

.072 

.073 

.074 

.076 

.077 

56.5 

.061 

.062 

.063 

.064 

.066 

.067 

.068 

.069 

.071 

.072 

.073 

.074 

.076 

.077 

.078. 

57 

.062 

.063 

.064 

.066 

.067 

.068 

.069 

.071 

.072 

.073 

.075 

'.076 

.077 

.078 

.080 

57.5 

.063 

.064 

.065 

.067 

'.068 

.069 

.071 

.072 

.073 

.075 

.076 

.077 

.078 

.080 

.081 

58 

.064 

.065 

.066 

.068 

.069 

.070 

.072 

.073 

.074 

.076 

.077 

.080 

.081 

.082 

58.5 

.065 

.066 

.068 

.069 

.070 

.072 

.073 

.074 

.076 

.077 

.078 

!o80 

.081 

.083 

.084 

59 

.066 

.067 

.069 

.070 

.072 

.073 

.074 

.076 

.077 

.078 

.080 

.  081 

.083 

.084 

.085 

59.5 

.067 

.068 

.070 

.071 

.073 

.074 

.075 

.077 

.078 

.080 

.081 

.'08^ 

.084 

.085 

.087 

*>  60 

.068 

.070 

.071 

.072 

.074 

.075 

.077 

.078 

.080 

.081 

.082 

.084 

.085 

.087 

.088 

60.5 

.069 

.071 

.072 

.074 

.075 

.076 

.078 

.079 

.081      .082 

.084 

.085 

.087* 

.088 

.089 

61 

.070 

.072 

.073 

.075 

.076     .078 

.079 

.081 

.  082      .  084 

.085 

.086 

.088 

.089 

.091 

61.5 

.071 

.073 

.074 

.076 

.  077     .  079 

.080 

.082 

.083      .085 

.086 

.088 

.089 

.091 

.092 

62 

.073 

.074 

.076 

.077     .079  j  .080 

.082 

.083 

.085      .086 

.088 

.089 

.091 

.092 

.094 

1 

62.5 

.074 

.075 

.077 

.078 

.080     .081 

.083 

.084 

.086 

.087 

.089 

.090 

.092 

.094 

.095 

63 

.075 

.076 

.078 

.079 

.  081      .  082 

.084 

.086 

.087      .089 

.090 

.092 

.093 

.095 

.096 

63.5 

.076 

.077 

.079 

.080 

.082  .  .084 

.085 

.087 

.088     .090 

.092 

.093 

.095 

.096 

.098 

64 

.077 

.078 

.080 

.082     .083     .085 

.086 

.088 

.090 

.091 

.093 

.094 

.096 

.098 

.099 

64.5 

.078 

.080 

.081 

.  083     .  084     .  086 

.088 

i  .089 

.091 

.093 

.094 

.096 

.097 

.099 

.101 

65 

.079 

.081 

.082 

.084.    .086     .087 

.089 

.091 

.092     .094 

.095 

JJU7 

.099 

.100 

.102 

65.5 

080 

.082 

.083 

.085 

.087  ;  .088 

.090 

.092 

.093 

.095 

.097 

.100 

.102 

.103 

66 
66.5 

.081 
.082 

.083 
.084 

.085 
.086 

.086     .088     .090 
.  087     .  089      .  091 

.091 
.093 

.093 
.094 

.095  ;  .096 
.096     .098 

.098 
.099 

.100 
.101 

.101 
.103 

.103 
.105 

.105 
.100 

67 

.083 

.085 

.087 

.089 

.090     .092 

.094 

.095 

.  097      .  099 

.101 

.102 

.104 

.106 

.108 

; 

1 

, 

BAROMETERS    AND  MKAsr  UK.MKXT   OF    ATMOSPHERIC    PRESSURE.        95 
TABLE  I.— Correction  of  mercurial  barometer  for  temperature,  English  measures— Con. 


Observed  reading  of  the  barometer,  in  inches. 

•F. 

17 

17.5 

18 

18.5 

19 

19.5 

20        20.5 

21 

21.5 

22 

22.5 

•->.;.  o 

24 

Inches. 

SUBTRACT.                                Inches. 

67 
67.5 

0.059 
.060 

0.061 
.062 

0.062  10.064 
.063     .065 

0.066  0.068 
.067     .069 

0.069 
.070 

.071 
.072 

0.073  '0.075 
.074     .076 

0.076  0.078  0.080  !o.082 
.077     .079     .081  '    row 

0.083 
084 

68 
68.5 

.061 
.061 

.062 
.063 

.064 
.065 

.066 
.067 

.068 
.069 

.069 
.070 

.071 
.072 

.073 
.074 

.075     .077 
.076.     .078 

.078     .080     .082 
.079     .081     .083 

.084 
.085 

1085 
.087 

69 

.062 

.064 

.066 

.068 

.069 

.071 

.073 

.075 

.077     .079 

.080     .082     .084 

.086 

.088 

69.5 

.063 

.065 

.067 

.068 

.070 

.-072 

.074 

.076 

.078     .079 

.081      .083     .085 

.087 

.089 

70 

.064 

.065 

.067 

.069 

.071 

.073 

.075 

.077 

.079     .080     .082  '<  .084     !086 

.088 

.090 

70.5 

.064 

.066 

.068 

.070 

.072 

.074 

.076 

.078 

..080     .081  ;  .083     .085     .087 

.089 

.on 

71 

.065 

.067 

.069 

.071 

.073 

.075 

.077 

.079 

.080     .082     .084     .086     .088 

.090 

092 

71.5 

.066 

.068 

.070 

.072 

.074 

.076 

.078 

.079 

.081     .083 

.085     .087     .089 

.091 

.093 

72 

.067 

.069 

.071 

.073 

.075 

.076 

.078 

.080 

.082     .084 

.086 

.088      .090 

.092 

.094 

72.$ 

.067 

.069 

.071 

.073 

.075 

.077 

.079 

.081 

.083     .085 

.087     .089 

.091 

.093 

.095 

73 

.068 

.070 

.072 

.074 

.076 

.078 

.080 

.082 

.084     .086 

.088     .090     .092 

.094 

.096 

73.5 

.069 

.071 

.073 

.075     .077 

.079 

.081 

.083 

.085  i  .087 

.089     .091     .  OM 

095 

097 

74 

.070 

.072 

.074 

.076 

.078 

.080 

.082 

.084 

.086     .088 

.090     .092 

.094 

.096 

.098 

74.5 

.070 

.073 

.075 

.077 

.079 

.081 

.083 

.085 

.087     .089 

.091     .093 

.095 

.097 

.100 

75 

.071 

.073 

.075 

.078 

.080 

.082 

.084 

.086 

.088     .090 

.092     .094 

.096 

.099 

.101 

75.5 

.072 

.074 

.076 

.078 

.081 

.083 

.085 

.087 

.089     .091 

.093     .095 

.097 

.100 

.102 

76 

.073 

.075 

.077 

.079 

.081 

.084 

.086 

.088 

.090     .092 

.094     .096 

.098 

.101 

.103 

76.5 

.074 

.076 

.078 

.080 

.082 

.084 

.087 

.089 

.091     .093 

.095 

.097 

.100 

.102 

.104 

77 

.074 

.077 

.079 

.081 

.083 

.085 

.087 

.090 

.092 

.094 

.096 

.098 

.101 

.103 

.105 

77.5 

.075 

.077 

.080 

.082 

.084 

.086 

.088 

.091 

.093     .095  !  .097 

.099 

.102 

.104 

.106 

78 

.076 

.078 

.080 

.083 

.085 

.087 

.089 

.091 

.094     .096  j  .098 

.100 

.103 

.105 

.107 

78.5 

.077 

.079 

.081 

.083 

.086 

.088 

.090 

.092 

.095     .097  |  .099 

.101 

.104 

.106 

.108 

79 

.077 

.080 

.082 

.084 

.086 

.089 

.091 

.093 

.096     .098 

.100     .102 

.105 

..107 

.109 

79.5 

.078 

.080 

.083 

.085 

.087 

.090 

.092 

.094 

.097     .099 

.101 

.103 

.106 

.108 

.110 

80 

.079 

.081 

.084 

.086 

.088 

.091 

.093 

.095 

.097     .100 

.102 

.104 

.107 

.109 

.111 

80.5, 

.080 

.082 

..084 

.087 

.089 

.091 

.094 

.096 

.098     .101 

.103 

.105 

.108 

.110 

.112 

81 

.080 

.083 

.085 

.088 

.090 

.092 

.095 

.097 

.099     .102 

.104 

.106 

.109 

.111 

.114 

8-1.5 

.081 

.084 

.086 

.088 

.091 

.093 

.096 

.098 

.  100     .  103 

.105 

.107 

.110 

.112 

.115 

82 

.082 

.084 

.087 

.089 

.092 

.094 

.096  : 

.099 

.101 

.104 

.106 

.108 

.111 

.113 

.116 

82.5 

.083 

.085 

.088 

.090 

.092 

.095 

.097  • 

.100 

.102 

.105 

.107 

.109 

.112 

.114 

.117 

83 

.083 

.086 

.088 

'.091 

.093 

.096 

.098 

.101 

.103 

.106 

.108 

.111 

.113 

.115 

.118 

83.5 

.084 

.087 

.089 

.092 

.094 

.097 

.099 

.102 

.104 

.107 

.109 

.112 

.114 

.117 

.119 

84 

.085 

.088 

.090 

.093 

.095 

.098 

.100 

.103 

.105 

.108 

.110 

.113 

.115 

.118 

.120 

84.5 

.086 

.088 

.091 

.093 

.096 

.098 

.101 

.103 

.106 

.108 

.111 

.114 

.116 

.119 

.121 

85 

.087 

.089 

.092 

.094 

.097 

.099 

.102 

.104 

.107 

.109 

.112 

.115 

.117 

.120 

.122 

85.5 

.087 

.090 

.092 

.095 

.098 

.100 

.103 

.105 

.108 

.110 

.113 

.116 

.118 

.121 

.123 

86 

.088 

.091 

.093 

.096 

.098 

.101 

.104 

.106 

.109 

.111 

.114 

.117 

.119 

.122 

.124 

86.5 

.089 

.091 

.094 

.097 

.099 

.102 

.105 

.107 

.110 

.112 

.115 

.118 

.120 

.123 

.125 

87 

.090 

.092 

.095 

.098 

.100 

.103 

.105 

.108 

.111.!  .113 

.116 

.119 

.121 

.124 

.126 

87.5 

.090 

.093 

.096 

.098 

.101 

.104 

.106 

.109 

.112     .114 

.117 

.120 

.122 

.125 

.128 

88 

.091 

.094 

.096 

.099 

.102 

.105 

.107 

.110 

.113     .115 

.118 

.121 

.123 

.126 

.129 

88.5 

.092 

.095 

.097 

.100 

.103 

.105 

.108 

.111 

.114 

.116 

.119 

.122 

.124 

.127 

.130 

•    89 

.093 

.095 

.098 

.101 

.104 

.106 

.109 

.112 

.114 

.117 

.120 

.123 

.125 

.128 

.131 

89.5 

.093 

.096 

.099 

.102 

.104 

.107 

.110 

.113 

.115     .118 

.121 

.124 

.126 

.129 

.132 

90 

.094 

.097 

.100 

.102 

.105 

.108 

.111 

.114 

.116     .119 

.122 

.125 

.127 

.130 

.133 

90.5 

.095 

.098 

.101 

.103 

.106 

.109 

.112 

.114 

.  117     .  120 

.123 

.128 

.131 

.134 

91 

.096 

.099 

.101 

.104 

.107 

.110 

.113 

.115 

M18     .121 

.124     ,181 

.129 

.132 

.135 

91.5 

.096 

.099 

.102 

.105 

.108 

.111 

.113 

.116 

.  119     .  122 

.125 

.128 

.131 

.133 

.136 

92 

.097 

.100 

.103 

.106 

.109 

.112 

.114 

.117 

.120     .123 

.126 

.129 

.132 

.134 

.137 

92.5 

.098 

.101 

.104 

.107 

.110 

.112 

.115 

.118 

.  121     .  124 

.127     .130 

.133 

.135 

.138 

93 

.099 

.102 

.105 

.107 

.110 

.113 

.116 

.119 

.122 

.125 

.128 

.131 

.  134 

.137 

.139 

93.5 

.100 

.102 

.105 

.108 

.111 

.114 

.117 

.120 

.123 

.126 

.129 

.132 

.135 

.138 

.140 

M 

.100 

.103 

.106 

.109 

.112 

.115 

.118 

.121 

.124 

.127 

.130 

.133 

.136 

.139 

.142 

94.5 

.101      .104 

.107 

.110 

.113 

.116  ^  .119 

.122 

.125 

.128 

.131 

.134 

.137 

.140 

.143 

95 

.102      .10.-, 

.108 

.111 

.114 

.117      .120 

.123 

.126 

.129 

.132 

.135 

.138 

.141 

.144 

95.5 

.103      .10., 

.109 

.112 

.115 

.118 

.121 

.124 

.127 

.130 

.133 

.136 

.139 

.142 

.145 

96 

.  103     .  106 

.109 

.112 

.115 

.119 

.122 

.125 

.128 

.131 

.134 

.137 

.140 

.143 

.146 

96.5 

.104     .107 

.110 

.113 

.116 

.119 

.122      .126 

.129 

.132 

.135 

.138 

.141 

.144 

.147 

97 

.105 

.108 

.111 

.114 

.117 

.120 

.123 

.126 

.130 

.133 

.136 

.139 

.142 

.145 

.148 

97.5 

.106 

.109 

.112 

.115 

.118. 

.121 

.124 

.127 

.130 

.134 

.137 

.140 

.143 

.146 

.149 

98 

.106     .109 

.113 

.116 

.119 

.122 

.125 

.131 

.135 

.138 

.141 

.144 

.147 

..150 

98.5 

.107      .110 

.113 

.117 

.120 

.123 

.126      .129 

.132 

.135 

.139 

.142 

.145 

.148 

.151 

99 

.108     .111 

.114 

.117 

.121 

.124 

.127       .130 

.133 

..136 

.140 

.143 

.146 

.149 

.  152 

99.5 

.109 

.112 

.115 

.118 

.121 

.125 

.128 

.131 

.1M4 

.137 

.141 

.144 

.147 

.150. 

100 

.109 

.113 

.116 

.119 

,189 

.1.'., 

.  129      .  132 

.135 

.138 

.  n.1 

.  1  »:, 

.148 

.151 

96        BAROMETERS   AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE. 
TABLE  I. — Correction  of  mercurial  barometer  for  temperature,  English  measures — Con' 


Observed  reading  of  the  barometer,  in  inches. 

f 

24 

. 
24.5 

25 

25.5 

26 

26.5 

27 

27.5 

IN       28.  5 

29       29.5  ,     30    I  30.5       31 

Inches.                              SUBTRACT.                               Inches. 

67 

0.083   0.085  0.087 

0.089 

0.090   0.092   0.094  j 

0.095 

0.097   0.099   0.101    0.102 

0.104 

0.106     0.108 

67.5 

.084 

086 

088 

090 

092       093      -095  I 

.097 

.  098     .  100 

.  102     .  104 

.106 

.107  '     .109 

68' 

!085 

'.087 

.089 

.091 

.093      .094 

.096 

.098 

.  100     .  102 

.103 

.105 

.107 

.109  ;     .110 

68.5 

087 

088 

090 

092 

094     .096 

.097 

.099 

.101 

.103 

.105 

.106 

.108 

.110  i     .112 

69' 

.088 

.089 

.091 

.093 

.095  I  .097 

.099 

.100 

.102 

.104 

.106 

.108 

.110 

.111       .113 

69.5 

.089 

091 

092 

094 

096       098 

.100 

.102 

.104 

.105 

.107 

.109 

.111 

.113  i     .115 

70' 

!092 

.094 

.095 

.097  1  .099 

.101 

.103 

.105 

.107 

.109' 

.110 

.112 

.114       .116 

70.5 

!091 

.093 

.095 

.097 

.098  !  .100 

.102 

.104 

.106 

.108 

.110 

.112 

.114 

.116       .117 

71 

.092 

.094 

.096 

.098 

.100  :  .102 

.103 

.105 

.107 

.109 

.111 

.113 

.115 

.117       .119 

71.5 

.093 

.095 

.097 

.099 

.101      .103 

.105 

.107 

.109 

.110 

.112 

.114 

.116 

.118       .120 

72 

.094 

.096 

.098 

.100 

.102  |  .104 

.106 

.108 

.110 

.112 

.114 

.116 

.118 

.  120       .  122 

72.5 

.095 

.097 

.099 

.101 

.  103     .  105 

.107 

.109 

.111 

.113 

.115 

.117 

.119 

.121       .123 

73 

.096 

.098 

.100 

.102 

.  104     .  106 

.108 

.110 

.112 

.114 

.T16 

.  118  1  .  120 

.122       .124 

73.5 

.097 

.099 

.101 

.103 

.105     .108 

.110 

.112 

.114 

.116 

.118 

.120 

.122 

.  124       .  126 

74 

.098 

..101 

.103 

.105 

.107     .109 

.111 

.113 

.115 

.117 

.119 

.121 

.123 

.125 

.127 

74.5 

.100 

.102 

.104 

.106 

.  108     .  110 

.112 

.114 

.116 

.118 

.120 

.122 

.124 

.126 

.129 

75 

.101 

.103 

.105 

.107 

.109 

.111 

.113 

.115 

.117 

.119 

.122 

.124 

.126 

.128 

.130 

75.5 

.102 

.104 

.106 

.108 

.110 

.112 

.114 

.117 

.119 

.121 

.123 

.125 

.127 

.129 

.131 

76 

.103 

.105 

.107 

.109 

.111 

.113 

.116 

.118 

.120 

.122 

.124 

.126 

.128 

.131 

.133 

76.5 

,104 

.106 

.108 

.111 

.113 

.115 

.117 

.119 

.121 

.123 

.125 

.128 

.130 

.132 

.134 

77 

.105 

.107 

.109 

.112 

.114 

.116 

.118 

.120 

.122 

.125 

.127 

.129 

.131 

.133 

.136 

77.5 

.106 

.108 

.110 

.113 

.115 

.117 

.119 

.121 

.124 

.126 

.128 

.130 

.133 

.135 

.137 

78 

.107 

.109 

.112 

.114 

.116 

.118 

.120 

.123 

.125 

.127 

.129 

.132 

.134 

.136 

.138 

78.5 

.108 

.110 

.113 

.115 

.117 

.119 

.122 

.124 

.126 

.128 

.131 

.133 

.135 

.137 

.140 

79 

.109 

.112 

.114 

.116 

.118 

.121 

.123 

.125 

.127 

.130 

.132 

.134 

.137 

.139 

.141 

79.5 

.110 

.113 

.115 

.117 

.120 

.122 

.124 

.126 

.129 

.131 

.133 

.136' 

.Ig8 

.140 

.143 

80 

.111 

.114 

.116 

.118 

.121 

.123 

.125 

.128 

.130 

.132 

.135 

.137 

.139 

.142 

.144 

80.5 

.112 

.115 

.117 

.120 

.122 

.124 

.127 

.129 

.131 

.134 

.136 

.138 

.141 

.143, 

.145 

81 

.114 

.116 

.118 

.121 

.123 

.125 

.128 

.130 

.132 

.135 

.137 

.140 

.142 

.144' 

.147 

81.5 

.115 

.117 

.119 

.122 

.124 

.127 

.129 

.131 

.134 

.  136     .  139 

.141 

.143 

.146 

.148 

82 

.116 

.118 

.121 

.123 

.125 

.128 

.130 

.133 

.135 

.  137      .  140 

.142 

.145 

.147 

.149 

82.5 

.117 

.119 

.122 

.124 

.127 

.129 

.131 

.134 

.136 

.  139     .  141 

.144 

.146 

.148 

.151 

83 

.118 

.120 

.123 

.125 

.128 

.130 

.133 

.135 

.138      .140     .  142 

.145 

.147 

.150 

.152 

83.5 

.119 

.121 

.124 

.126 

.129 

.131 

.134 

.136 

.139 

.  141      .  144 

.146 

.149 

.151 

.154 

84 

.120 

.123 

.125 

.128 

.130 

.133 

.135 

.138 

.140 

.  143      .  145 

.148 

.150 

.153 

.155 

84.5 

.121 

.124 

.126 

.129 

.131 

.134 

.136 

.139 

.141 

.144 

.146 

.149 

.151 

.154 

.156 

85 

.122 

.125 

.127 

.130 

.132 

.135 

.137 

.140 

.143 

.145 

.148 

.150 

.153 

.155 

158 

85/5 

.123 

.126 

.128 

.131 

.134 

.136 

.139 

.141 

.144 

.146 

.149 

.152 

.154 

.157 

.159 

86 

.124 

.127 

.130 

.132 

.  135     .  137 

.140 

.142 

.145 

.148 

.150 

.153 

.155 

.158 

.161 

86.5 

.125 

.128 

.131 

.133 

.  136     .  138 

.141 

.144 

.146 

.149 

.152 

.154 

.157 

.159 

.162 

87 

.126 

.129 

.132 

.134 

.137 

.140 

.142 

.145 

.148 

.150 

.153 

.155 

.158 

.161 

.163 

87.5 

.128 

.130 

.133 

.136 

.138 

.141 

.144 

.146 

.149 

.151 

.154 

.157 

.159 

.162 

.165 

88 

.129 

.131 

.134 

.137 

.139 

.142 

.145 

.147 

.150 

.153 

.155 

.158 

.161 

.163 

.166 

88.5 

.130 

.132 

.135 

.138 

.141 

.143 

.146 

.149 

.151 

.154 

.157 

.159 

.162 

.165 

.168 

89 

.131 

.134 

.136 

.139 

.142 

.144 

.147 

.150 

.153  !  .155 

.158 

.161 

.164 

.166 

.169 

1 

89.5 

.132 

.135 

.137 

.140 

.143 

.146 

.148 

.151 

.154 

.157 

.159 

.162 

.165 

.168 

.170 

90 

.133 

.136 

.138 

.141 

.144 

.147 

.150 

.152 

.155- 

.158 

.161 

.163 

.166 

.169 

.172 

90.5 

.134 

.137 

.140 

.142 

.145  j  .148 

.151 

.154 

.156 

.159 

.162 

.165 

.168 

.170 

.173 

91 

.135 

.138 

.141 

.144 

.146 

.149 

.152 

.155 

.158 

.160 

.163 

.166 

.169 

.172 

.175 

91.5 

.136 

.139 

.142 

.145 

.  148     .  150 

.153 

.156 

.159 

.162 

.165 

.167 

.170 

.173 

.176 

92 

.137 

.140 

.143 

.146 

.  149     .  152 

.154 

.157 

.160 

.163 

.166 

.169 

.172 

.174 

.177 

92.5 

.138 

.141 

.144 

.147 

.150 

.153 

.156 

.159 

.161 

.164 

.167 

.170 

.173 

.176 

.179 

93 

.139 

.142 

.145 

.148 

.  151      .  154 

.157 

.160 

.163 

.166 

.168 

.171 

.174 

.177 

.180 

93.5 

.140 

.143 

.146 

.149 

.152 

.155 

.158 

.161 

.164 

.167 

.170 

.173 

.176 

.179 

.181 

94 

.142 

.145 

.147 

.150 

.153 

.156 

.159 

.162 

.165 

.168 

.171 

.174 

.177 

.180 

.183 

94.5 

.143 

.146 

.149 

.152 

.155 

.158 

.160 

.163 

.166 

.169 

.172 

.175 

.178 

.181 

.184 

95 

.144 

.147 

.150 

.153 

.156 

.159 

.162 

.165 

.168 

.171 

.174 

.177 

.180 

.183 

.186 

95.5 

.145 

.148 

.151 

.154 

.157 

.160 

.163 

.166 

.169 

.172 

.175 

.178 

.181 

.184 

.187 

96 

.146 

.149 

.152 

.155 

.158 

.161 

.164 

.167 

.170 

.173 

.176 

.179 

.182 

.185 

.188 

96.5 

.147 

.150 

.153 

'.156 

.159 

.162 

.165 

.168 

.171 

.174 

.178 

.181 

.184 

.187 

.190 

97 

.148 

.151 

.154 

.157 

.160 

.163 

.167 

.170 

.173 

.176 

.179 

.182 

.185 

.188 

.191 

97.5 

.149 

.152 

.155 

.158 

.162 

.165 

.168 

.171 

.174 

.177 

.180 

.183 

.186 

.189 

.193 

98 

.150 

.153 

.156 

.160 

.163 

.166 

.169 

.172 

.175 

.178 

.181 

.185 

.188 

.191 

.194 

98.5 

.151 

.154 

.158 

,161 

.164 

.167 

.170 

.173 

.176 

.180 

.183 

.186 

.189 

.192 

.195 

99 

.152 

.155 

.159 

.162 

.165 

.168 

.171 

.175 

.178 

.181 

.184 

.187 

.190 

.194 

.197 

99.5 

.153 

.157 

.160 

.163 

.166 

.169 

.173 

.176 

.179 

.182 

.185 

.189 

.192 

.195 

.198 

100 

.154 

.158 

.161 

.164 

.167 

.171 

.174 

.  177     .  180 

.183 

.187 

.190 

.193 

.196 

.200 

BAROMETERS   AND  MEASUREMENT   OF   ATMOSPHERIC    PRESSUKK. 
TABLE  II.— Correction  of  mercurial  barometers  for  temperature,  metric  meatureg. 
Observed  reading  of  the  barometer,  in  millimeters. 


,ure, 
C. 

640 

650 

660 

670 

680       690 

700 

7W 

720       730 

740 

750 

760        770 

0.0 
1.0 
2.0 

0.00 
.10 

.21 

0.00 
•  11 

.21 

0.00     0.00 
.11       .11 

0.00     0.00 
.11       .11 
.22       .23 

0.00 

:£ 

0.00 
.12 
.23 

0.00     0.00 
•  12       .12 
.24       .24 

0.00 
.12 
.24 

0.00 
.12 

0.00     0.00 
.12       .13 
.25       .25 

II.  IN' 

.13 
.25 

3.0 

.31 

.32 

.32       .33       .33       .34 

.34 

.35 

.35       .36 

.36 

.37 

.37        38 

00 

4.0 

.42 

.42 

.43       .44 

.44       .45 

.46 

.46 

.47       .48 

.48 

.49 

.60       .60 

.  oo 
.51 

5. 

.52 

.53 

.54 

.55 

.56       .56 

.57 

.58 

.59       .60       .60 

.01 

.62       .63 

.64 

5.5 

.57 

.58 

.59 

.«)       .61       .62       .»« 

.64 

.65       .66 

.66 

.67 

.68       .69 

70 

ii. 

.63 

.64 

.65 

.66 

.67       .68       .69 

.70 

.71       .71 

.72 

.73 

.  74       .  75 

.  *u 
.76 

8.1 

.68 

.69 

.70       .71 

.72       .73       .74 

.75 

.76 

.77 

.79 

.so 

.81       .82 

~- 

.73 

.74 

.75       .77 

.78       .79 

.80 

.81 

.82 

.83 

.85 

.86 

.87       .88 

'.89 

7.  ") 

.78 

.80 

.81       .82 

.83       .84       .86 

.87 

.88 

.VI 

.91 

.92 

.93       .94 

.95 

8. 

.84 

.85 

.86       .87       .89       .-90       .91        .93 

.94 

.97 

.98 

.99     1.01 

1.02 

8.5 

.89 

.90 

.92 

.93       .94 

.96 

.97 

1.00 

i!oi 

1.03 

1.04 

1.05 

1.07 

1.08 

9. 

.94       .95 

.97 

.98     1.00 

1.01 

1.03 

1.04 

1.06 

1.07 

1.09 

1.10 

1.12 

1.  13 

1.  15 

9.5 

.99     1.01 

1.02 

1.04 

1.05 

1.07      1.08 

1.10 

1.12 

1.13 

1.15 

1.16 

1.18 

1.19 

1.21 

10.0 

1.04     1.06 

1.08 

1.09 

1.11 

1.13 

1.14       l.hi 

1.17 

1.19 

1.21 

1.22 

1.24 

1.2', 

1.27 

.2 

.06       .08 

.10 

.11 

.13 

.15 

.H,        .18 

.20 

.21 

.23 

.26       .28 

.30 

.4 

.09       .10 

.12 

.14 

.15 

.17 

.19        .20 

.22 

.  21 

.26 

.27 

.29       .31 

.32 

.6 

.11  "    .12 

.14 

.16 

.18 

.19 

.21        .23 

.24 

.28 

.30 

.31 

.33 

.35 

.8 

.13       .14 

.16 

.18 

.20 

.22 

.23      ..26 

.27 

!  ~".i 

.30 

.32 

.34       .36 

.37 

11.0 

1.15      1-17 

1.18 

1.20 

1.22 

1.24      1.26     -1.27 

1.29 

1.31 

1.33 

1.35 

1.36     1.38 

1.42 

.2 

.17       .19 

.21 

.22 

.24 

.2ii        .28 

.30 

.31       .33 

.35 

.37 

.39       .41 

.45 

.4 

.19        .21 

.23 

.25 

.26 

.28       .30 

.32 

.34       .36       .38 

.39 

.41 

.40 

.6 

.  21        .  23 

.25 

.27 

.29 

.31       .32 

.34 

.36 

.38 

.40 

.42 

.44 

•  48 

.8 

.23 

.25 

.27 

.29 

.31 

.33 

.35 

.37 

.39 

.40 

.42 

.44 

.46 

.48 

.50 

12.0 

1.25      1.27 

1.29 

1.31 

1.33 

1.35 

1.37 

1.39 

1.41 

1.43 

1.45 

1.47 

1.49 

1.51 

1.53 

.2 

.27  '     .29 

.31 

.33 

.35       .37 

.39        .41 

.43 

.45 

.47 

•  49 

.51 

.63 

.55 

.4 

.29       .31 

.33 

.35 

.37 

.39 

.44 

.46       .48 

.50 

.52 

.64 

.66 

.68 

.6 

.31       .34 

.36 

.38 

•  40 

.42 

.44 

.46 

.48       .50 

.52 

.54 

.66 

.60 

•& 

.34 

.36 

.38 

.40 

.42 

.44 

.46 

.48 

.50 

.52 

.54 

.56 

.59 

la 

.63 

13.0 

1.36     1.38 

1.40 

1.42 

1.44 

1.46 

1.48 

1.50 

1.  53     1.  55 

1.57 

1.59 

1.61 

LM 

1.65 

.2 

.38       .40 

.42 

.44 

.46 

.48       .51        .53 

.57       .59 

.61 

.64  ;     .66 

.68 

.4 

.40       .42 

.44 

.46 

.49 

,j]        .53        .55 

.  .-,7       .  59 

.62 

.64 

.66 

.fc 

.70 

.6 

.42       .44 

.46 

.49 

.51 

.53 

.55 

.57 

.60 

.62 

.64 

.66 

.68 

.71 

.73 

.8 

.44 

.46 

.48 

.51 

.53 

.55 

.57 

.60 

.62 

.64 

.66 

.69 

.71 

.73 

.75 

14.0 

1.46     1.48 

1.51 

1.53 

1.55 

1.57 

1.60 

1.62 

1.64 

1.67 

1.69 

1.71 

1.73 

1.76 

1.78 

.2 

.48       .50 

.53 

.55 

.o7 

.60       .62        .64 

.67 

.69 

.71 

.74 

.76 

!78 

.81 

.4 

.50       .53 

.55 

.57 

.60 

.62       .64        ,87 

.69 

.71 

.74 

.76 

.78 

.81 

.83 

.  ti 

.  52 

.57 

.59 

.62 

.64 

.67 

.69 

.71 

.74 

.76 

.78 

.81 

.83 

.86 

.8 

.54 

.57 

.59 

.62 

.64 

.66 

.69 

.71 

.74 

.76 

.78 

.81 

.83 

.86 

.88 

15.0 

1.56     1.59 

1.61 

1.64 

1.66 

1.69 

1.71 

1.74 

1.76 

1.78 

1.81 

1.83 

1.86 

1.88 

1.91 

.2 

.59       .61 

.63 

.68 

.71 

.73 

.76 

.78 

.81 

.83 

.86 

.88 

.91 

.93 

.4 

.61       .63 

.66 

.68       .71 

.76 

.78 

.81 

.83 

.86 

.88 

.91       .93 

.96 

.1; 

.63       .65 

.68 

.70       .73       .75 

.78 

.80 

.83 

.86 

.88 

.91 

.93       .96 

.98 

.8 

.65       .67 

.70 

.  72       .  75 

.78 

.80 

.83 

.85 

.88 

.90 

.93 

.96  |    .98 

2.01 

!.,.() 

1.67     1.69 

1.72 

1.75     1.77 

1.80 

1.82 

1.85 

1.88 

1.90 

1.93 

1.96 

1.98     2.01 

2.03 

.69       .72 

.74 

.77       .79 

.82 

.85 

.87 

.90       .93 

.95 

.98 

2.01 

.03 

.06 

.4 

.71       .74 

.76 

.79       .82 

.84 

.87        .90 

.92 

.  95 

.98 

2.00 

.03 

.06 

.06 

.6 

.73       .76 

.78 

.81 

.84 

.87 

.89        .92 

.95 

.97 

2.00 

.03 

.06 

.06 

.11 

.8 

.75       .78 

.81 

.83 

.86 

.89 

.92 

.94 

.97 

2.00 

.03 

.05 

.08 

.11 

.13 

17.0 

1-.77     1.80 

1.83 

1.86     1.88 

1.91      1.94  f 

1.97 

1.99     2.02 

2.05 

2.08 

2.10 

2.13 

2.16 

9 

.79       .82 

.85 

.88       .91       .93       .96  1!     .99 

2.02       .05 

.07 

.10       .13       .16 

.19 

^4 

.81       .84 

.87 

.90       .93       .96       .98      2.01 

.04       .07 

.10 

.13       .15       .18 

.21 

.6 

.83       .86 

.89 

.92       .95 

.98     2.01        .04 

.06 

.09 

.12 

.15       .18 

.21 

.24 

.8 

.86       .88 

.91 

.94       .97 

2.00       .03        .06 

.09 

.12 

.15 

.17       .20 

.23 

.26 

18.0 

1.88 

1.91 

1.93 

1.96     1.99     2.02     2.05      2.08 

2.11 

2.14 

2.17 

2.20 

2.23 

2.26 

2.29 

.2 

.90 

.93 

.96 

.99     2.02  !     .05       .07           H) 

.13 

.16 

.19 

.22       .25 

.28 

.31 

.4 

.  '.12 

"  it:, 

.98 

2.01       .04       .07       .10        .13 

.If, 

.19 

.22 

.25       .28 

.31 

.M 

.6 

.94       .97 

2.00 

.03       .06       .09         12 

.15 

,u 

.21 

.24 

.27       .30 

.8 

.96       .99 

.02 

.05       .08       .11       .14 

.17 

.23 

.27 

.30       .33 

19.0 

1.98     2.01 

2.04 

2.07     2.10     2.13     2.17 

2.20 

2.29 

2.32     2.35     2.38 

2.41 

.2 

2.00       .03 

.06 

.09       .13       .10       .19 

.28 

.31 

.34       .38       .41 

.44 

.4 

.02       .05 

.08 

.12        .15       .18        .21         .24 

.17        .31 

.34 

.37        .40       .43 

.4f. 

.6 

.04       .07 

.11 

.14        .17        .20       .23 

.27 

.30 

.33 

.36 

.39 

.46 

.49 

.8 

.06       .10 

.13 

.16       .19       .22       .26 

.29 

.35 

.39 

.42       .45 

.51 

22937—12 7 


98        BAROMETERS    AND   MEASUREMENT    OF    ATMOSPHERIC    PRESSURE.    • 
TABLE  II. — Correction  of  mercurial  barometers  for  temperature,  metric  measures — Con. 


Observed  reading  of  the  barometer,  in  millimeters. 

640 

650 

660 

670 

680 

690 

700 

710 

720 

730 

740 

750 

760 

770 

2.06     2.10 

2.13 

2.16 

2.19 

2.22 

2.26 

2.29 

2.32 

2.35 

2.39 

2.42 

2.45 

2.48 

2.08 

2.12 

2.15 

2.18 

2.21 

2.25 

2.28 

2.31 

2.34 

2.38 

2.41     2.44 

2.47 

2.51 

.10 

.14 

.17 

.20 

.24 

.27 

.30 

.33 

.37 

.40 

.43       .47 

.50       .53 

.13 

.16 

.19 

.23 

.26 

.29 

.32 

.36 

.39       .42       .46       .49 

.52       .56 

.15 

.18 

.21 

.25 

.28 

.31 

.35 

.38 

.41        -45       .48       .52 

.55       .58 

.17 

.20 

.23 

.27 

.30 

.34 

.37 

.40 

.44       .47       .51       .54 

.57       .61 

2.19 

2.22 

2.26 

2.29 

2.32 

2.36 

2.39 

2.43 

2.46     2.50     2.53     2.56 

2.60     2.63 

.21 

.24 

.28 

.31 

.35 

.38 

.42 

.45 

.48       .52       .55       .59 

.62       .66 

.23 

.26 

.30 

.33 

.37 

.40 

.44 

.47 

.51       .54       .58       .61       .65       .68 

.25 

.29 

.32 

.36 

.39 

.43 

.46 

.50 

.53       .57       .60       .64 

.67       .71 

.27 

.31 

.34 

.38 

.41 

.45 

.48 

.52 

.55       .59       .63       .66 

.70 

.73 

2.29 

2.33 

2.36 

2.40 

2.43 

2.47 

2.51 

2.54 

2.58     2.61     2.65     2.69 

2.72 

2.76 

.31       .35 

.38 

.42 

.46 

.49       .53 

.57 

.60       .64       .67       .71       .75 

.78 

.33 

.37 

.41 

.44       .48       .52       ,55 

.59 

.62       .66 

.70       .73       .77 

.81 

.35 

.39 

.43 

.46 

.50       .54 

.57 

.61 

.65       .68 

.72       .76       .80 

.83 

.37 

.41 

.45 

.49 

.52 

.56 

.60 

.63 

.67       .71       .75       .78 

.82 

.86 

2.40 

2.43 

2.47 

2.51 

2.54 

2.  58  . 

2.62 

2.66 

2.69     2.73 

2.  77     2.  81      2.  84 

2.88 

.42 

.45 

.49 

.53 

.57 

.60 

.64 

.68 

.72 

.76 

.79 

.83 

.87 

.91 

.44 

.47 

.51 

.55 

.59 

.63 

.67 

.70 

.74 

.78 

.82 

.86       .89 

.93 

.46 

.50 

.53 

.57 

.61 

.65 

.69 

.73 

.76 

.80       .84 

.88  '     .92 

.96 

.48 

.52 

.56 

.59 

.63       .67 

.71 

.75 

.79 

.83       .87       .90  ;     .94 

.98 

2.50 

2.54 

2.58 

2.62 

2.66 

2.69 

2.73 

2.77 

2.81 

2.  85     2.  89 

2.93  j  2.97 

3.01 

.52 

.56 

.60 

.64 

.68 

.72 

.76 

.80 

.83 

.87       .91 

.95       .99 

.03 

.54 

.58 

.62 

.66 

.70 

.74 

.78 

.82 

.86 

.90       .94 

.98 

3.02 

.06 

.56 

.60 

.64 

.68 

.72 

.76 

.80 

.84 

.88 

.92       .96 

3.00 

.04 

.08 

.58 

.62 

.66 

.70 

.74 

.78 

.82 

.86 

.90 

.94       .99 

.03 

.07 

.11 

2.60 

2.64 

2.68 

2.72 

2.77 

2.81 

2.85 

2.89 

2.93 

2.97     3.01 

3.05 

3.06 

3.13 

.62 

.66 

.71 

.75 

.79 

.83       .87 

.91 

.95 

.99       .03 

.07 

.12 

.16 

.64 

.69 

.73 

.77 

.81 

.85       .89 

.93 

.97 

3.  02       .06       .  10 

.14 

.18 

.66 

.71 

.75 

.79 

.83 

.87       .91 

.96 

3.00 

.04       .08       .12 

.16 

.21 

.69 

.73 

.77 

.81 

.85 

.90 

.94 

.98 

.02 

.06       .11       .15 

.19 

.23 

2.71 

2.75 

2.79 

2.83 

2.88 

2.92 

2.96 

3.00 

3.04 

3.09 

3.  13     3.  17 

3.21 

3.26 

.73 

-77 

.81 

.85 

.90 

.94 

.98 

.03 

.07       .11 

.15       .20 

.24 

.28 

.75       .79 

.83 

.88 

.92 

.96 

3.01 

.05 

.09 

.13 

.18       .22 

.26 

.31 

.77 

.81 

.85 

.90 

.94 

.99 

.03 

.07 

.11 

.16 

.20       .24 

.29 

.33 

.79 

.83 

.88 

.92 

.96 

3.01 

.05 

.09 

.14 

.18 

.22       .27       .31 

.36 

2.81' 

2.85 

2.90 

2.94 

2.99 

3.03 

3.07 

3.12 

3.16 

3.20 

3.25 

3.29 

3.34 

3.38 

.83 

.87 

.92 

.96 

3.01 

.05 

.10 

.14 

.18       .23 

.27       .32 

.36 

.41 

.85 

.90 

.94 

.98 

.03 

.07 

.12 

.16 

.21       .25       .30       .34       .39 

.43 

.87 

.92 

.96 

3.01 

.05 

.10 

.14 

.19 

.23       .28       .32       .37       .41 

.46 

.89 

.94 

.98 

.03 

.07 

.12 

.16 

.21 

.25       -30       .34       .39       .43 

.48 

2.91 

2.96 

3.00' 

3.05 

3.10 

3.14 

3.19 

3.23 

3.28     3.32     3.37 

3.41 

3.46 

3.51 

.93       .98 

.03 

.07 

.12 

.16 

.21 

.25 

.30       .35       .39 

.44 

.48 

.53 

.95     3.  00 

.05 

.09 

.14 

.19 

.23 

.28 

.32       .37       .42 

.46       .51 

.56 

.98 

.02 

.07 

.11 

.16 

.21 

.25 

.30 

.35  '     .39       .44 

.49 

.53 

.58 

3.00 

.04 

.09 

.14 

.18 

.23 

.28 

.32 

.37       .42       .46 

.51 

.56 

.60 

3.02     3.06 

3.11 

3.16 

3.21 

3.25 

3.30 

3.35 

3.39 

3.44     3.49 

3.54 

3.58 

3.63 

.04       .08 

.13 

.18       .23 

.27 

.32 

.37 

.42       .46       .51 

.56       .61 

.65 

.06       .11 

.15 

.20 

.25 

.30 

.34 

.39 

.44  i     .49       .54 

.58       .63 

.68 

.08  1    .13 

.18 

.22 

.27 

.32 

.37 

.42 

.46       .51       .56 

.61       .66 

.70 

.10       .15 

.20 

.24       .29 

.34 

.39 

.44 

.49       .54       .58 

.63       .68 

.73 

3.12 

3.17 

3.22 

3.27     3.32 

3.36 

3.41 

3.46 

3.51 

3.56 

3.61 

3.  66     3.  71 

3.75 

.14 

.19 

.24 

.29       .34 

.39 

.44 

.48 

.53 

.58 

.63 

.  68       .73 

.78 

.16  !    .21 

.26 

.31       .36 

.41 

.46 

.51 

.56 

.   -61 

.66 

.71       .75 

.80 

.18 

.23 

.28 

.33       .38 

.43 

.48 

'.53 

.58 

.63 

.68 

.73       .78 

.83 

.20       .25 

.30 

.35       .40 

.45       .50 

.55 

.60 

.65 

.70 

.75       .80 

.85 

3.22 

3.27 

3.32 

3.37     3.43 

3.48     3.53 

3.58 

3.63 

3.68 

3.73 

3.78  i  3.83 

3.88 

.28  I     .33 

.38 

.43       .48 

.53       .58 

.63 

.68 

.74 

.79 

.84       .89 

.94 

.33 

.38 

.43 

.48       .54 

.59       .64 

.69 

.74 

.79 

.85 

.90  !     .95 

4.00 

.38 

.43 

.48 

.54       .59 

.64 

.70 

.75 

.80 

.85 

.91 

.96 

4.01 

.07 

.43 

.48 

.54 

.59       .64 

.70 

.75 

.81 

.86 

.91 

.97 

4.02 

.07 

.13 

3.48 

3.54     3.59 

3.65     3.70 

3.75 

3.81 

3.86 

3.92 

3.97 

4.03 

4.  08     4.  13 

4.19 

.53 

CQ 

.64 

.70       .75 

.81 

.87 

.92 

.98 

4.03 

.09 

.14       .20 

.25 

.59 

if>4 

.70 

.75       .81 

.87 

.92 

.98 

4.03 

.09 

.15 

.  20       .26 

.31 

.64 

.69 

.75 

.81       .86 

.92 

.98 

4.03 

.09 

.15 

.21 

.26  i     .32 

.38 

.69 

.75 

.80 

.86       .92 

.98 

4.03 

.09 

.15 

.21 

.26 

.32       .38 

.44  ' 

780 


BAROMETERS    AND   MEASUREMENT    OF   ATMOSPHERIC    PRESSURE.        99 


TABLE  III.—  Influence  of  gravity  on  barometric  observations,  English  units. 
[Abridged  from  International  Tables.] 
REDUCTION  TO  LATITUDE  45°. 


10 
11 
12 
LI 

14 

15 

16  N 
17 
18 
19 

20 

a 

22 
2:-! 
M 

25 

• 
27 
28 
29 

M 

n 

32 

n 


de. 

Reading  of  the  barometer,  in  inches. 

18 

19 

20 

21 

22 

23           24 

25         26 

27 

28 

N 

90 

0.047 

0.049 

0.052 

0.054 

0.057 

0.060     0.062 

0.065  0.067 

0.070 

0.073 

0.075     0.07S 

85 

.046 

.048 

.051 

.054 

.056 

.059       .061 

.064     .066 

.069     .071 

.  074       .  077 

84 

.046 

.048       .051 

.053 

.056 

.058       .061 

.063     .066 

.068     .071 

.073       .076 

83 

.045       .048       .050 

.053 

.055 

.058       .060 

.063     .065  '  .068 

.070 

.  073       .  075 

82 

.046       .047       .050 

.052       .055 

.057       .060 

.062     .065      .067 

.070 

.  072       .  07  j 

81 

.044       .047       .049 

.052 

.054 

.057       .059 

.062     .064 

.067     .069 

.071       .074 

80 

.044       .046       .  049 

.051 

.054 

.056       .058 

.061     .063 

.066 

.068 

.  071       .  073 

79 
78 

.043 
.043 

.046       .048 
.045       .047 

.050 
.050 

.053 
.052 

.055       .058 
.054       .057 

.060     .062 

.  059     .  Ot)2 

.065     .067 
.064     .066 

.070       .072 
.069       .071 

77 

.042 

.044       .047 

.049       .051 

.054       .056 

.058     .061      .063     .065 

.068 

76 

.  041        .043       .046 

.048       .050 

.053       .055 

.057     .059     .062     .064 

.066       .069 

75 

.040       .043       .045 

.047 

.049 

.052       .054 

.956     .058     .061     .063^ 

*.065       .067 

74 

.040        .042        .044 

.046 

.048 

.051       .053 

.055      .057  i  .059     .06F 

.064       .066 

73 

.  039        .  041        .  043 

.045       .047 

.049       .052 

.054     .056     .058     .060 

.062  .     .064 

72 

.038       .040 

.042 

.044 

.046 

.048       .050 

.052 

.054     .057 

.059 

.061       .063 

71 

.  037        .  039 

.041 

.043 

.045 

.047       .049 

.051 

.053 

.055 

.057 

.059       .061 

70 

.  036 

.038 

.040 

.042 

.044 

.046       .048 

.050 

.052 

.054 

.056 

.058       .060 

69 

.035 

.037 

.038 

.040 

.042 

.044       .046 

.048 

.050 

.052 

.054 

.056       .058 

68 

.034 

.035 

.037 

.039 

.041 

.043       .045 

.047 

.048 

.050 

.052 

.054       .066 

67 

.032  !     .034 

.036 

.038 

.040 

.041     .  .043 

.045 

.047 

.049 

.050 

.052       .054 

66 

.031 

.033 

.035 

.036 

.038 

.040       .042 

.043 

.045 

.047 

.049 

.050       .052 

65 

.030 

.032 

.033 

.035 

.037 

.038       .040 

.042 

.043 

.045 

.047 

.048       .050 

64 

.029 

.030 

.032 

.033 

.035 

.037       .038 

.040 

.041 

.043 

.045 

.046       .048 

63       .027 

.029 

.030 

.032 

.033 

.035       .037 

.038 

.040 

.041 

.043 

.044       .046 

62       .026 

.028 

.029 

.030 

.032 

.033       .035 

1008 

.038 

.039 

.041 

.042       .043 

61        .025 

.080 

.027       .029 

.030 

.032       .033 

.034 

.036 

.037 

.038 

.040       .041 

60       .OJH 

.025 

.026       .027 

.028 

.030       .031 

.032 

.034 

.035 

.036 

.038       .039 

59       .022 

.023 

.024       .026 

.027 

.028       .029 

.030 

.032 

.033 

.034 

.035       .036 

58       .020 

.022 

.023       .024 

.025 

.  026       .  027 

.028 

.030 

.031 

.032 

.033       .034 

57       .019 

.020 

.021       .022 

.023 

.  024       .  025 

.026 

.027 

.028 

.029 

.031       .032 

56       .  017 

.018 

.  019       .  020 

.021 

.022       .023 

.024 

.025 

.026 

.027 

.028       .029 

oo       .  016 

.017 

.018       .019 

.019 

.  020       .  021 

.022 

.023 

.024  ,  .025 

.026       .027 

54       .  014 

.01.5 

.OK)       .017 

.018 

.018       .019 

.020 

.021 

.022  '  .022 

.023       .024 

53  ,     .013 

.014 

.014       .015 

.016 

.016       i017 

.018 

.019 

.019,  .020 

.021       .021; 

52       .  Oil 

.012 

.013       .013 

.014 

.014       .015 

.016 

.016 

.017     .018 

.018'  .om 

51       .010 

.010 

.011        .011 

.012 

.  012       .  013 

.013 

.014 

.015     .015 

.016       .016 

50       .008 

.009 

.009       .009 

.010 

.010       .011 

.011 

.012 

.012 

.013 

.013  '    .013 

49  .     .006 

.007 

.007       .008 

.008 

.008       .009 

.009 

.009 

.010 

!oio 

.010       .011 

48       .005 

.005 

.005       .006 

.006 

.006       .006 

.007 

.007 

.007 

.008 

.008       .008 

47       .  003 

.003 

.004       .004 

.004 

.004       .004 

.005 

.005 

.005 

.006 

,008      .001 

46 

.002 

.002 

.002       .002 

.002 

.002       .002 

.002 

.002 

.002 

.003 

.003'     .003 

II 

.000 

.000 

.000       .000 

.000 

.000       .000 

.000 

.000 

.000 

.000 

.000       .00)) 

-Ob  * 


100      BAROMETERS  AND  MEASUREMENT   OF   ATMOSPHERIC   PRESSURE. 

TABLE  IV. — Influence^of  gravity  on  barometric  observations,  English  units. 

[From  International  Tables.] 

REDUCTION  FOR  ALTITUDE. 

To  be  subtracted. 


Height  in 
feet. 

Reading  of  the  barometer,  in  inches. 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27         28         29 

30- 

31 

0 
500 
1,000 
1,500 
2,000 

2,500 
3,000 
3,500 
4,000 
4,500 

5,000 
5,500 
6,000 
6,500 
7,000 

7,500 
8,000 
8,500 

9;  ooo 

9,500 
10,000 

0.000   0.000   0.(XKL 
.  001     .  001  K  001 
.002     .(JO'S  i  .002 
.002     .003     .003 
.003     .003     .003 

0.000 

Tooi 

.002 
.003 
.004 

0.000 
.001 
.002 

l  

0.002 
.003 



0.004     .004 
.004     .005 
.005      .005 
.006  !  .006 
.007  i  .007 

.004     .004     .004 
.005      .005      .005 
.006      .006    
.006     .007 





0.005 
.006 
.006 

0.006 

007 

.007 

.008 
.008 
009 

.007 
.008 
.009 
009 

.007 
.008 
.009 
.010 

.008 
.009 
.009 

.008    



6  (X)8 

.008 
009 







.009 

.009 
.010 
.011 
.011 
.012 

.013 

.009 

.010 
.011 
.011 
.012 
.012 

.013 

.010 

.010 
.011 
.012 
.012 

.010 

.011 
.011 

.010 

o.'oio" 

.011 

o.'oio" 

.010 
.011 

.011 

0.009 
.010 
.010 
.011 
.011 

.012 

------  •--•-•  -  

------ 

i 

TABLE  V. — Pressure,  in  inches,  corresponding  to  changes  of  100  feet  in  elevation. 

[BlGELOW.] 


Height,  in 

Temperature,  Fahrenheit. 

feet. 

i              j 

-20       -10 

0 

10 

20 

30 

40 

50 

60 

70           80      i      90  _ 

0 

0.128     0.125 

0.122 

0.119 

a.  117 

0.114 

0.112 

0-HP.L 

0.108 

0.106 

0.104 

0.102 

500 

.  125       .  122 

.120 

.117 

.115 

.112 

.110 

.108 

**.106 

.104 

,102 

.100 

1,000 

.  122       .  120 

.118 

.115 

.113 

.110 

.108 

.106 

.104 

.102 

.100 

.098 

1,500 

.  120       .  117 

.115 

.112 

.110 

.108 

.106 

.104 

.102 

.100 

.098 

.096 

2,000 

.118       .115 

.113 

.110 

.108 

.106 

.104 

.102 

.100 

.098 

.096 

.094 

2,500 

.115       .112 

.110 

.108 

.106 

.104 

.102 

.100 

.  .098 

.096 

.094 

.092 

3,000 

.113       .110 

.108 

.106 

.104 

.102 

.100 

.098 

.096 

.094 

.092 

.090 

3,500 

.111       .108 

.106 

.104 

.102 

.100 

.098       .096 

.094 

.092 

.090 

.088 

4,000 

.  108       .  106 

.104 

.102 

.100 

.098 

.096 

.094 

.092 

.090 

.088 

.086 

4,500 

.  106       .  104 

.102 

.100 

.098 

.096 

.094 

.092 

.090 

.088 

.086 

.084 

5,000 

.  104       .  102 

.100 

.098- 

.096 

.094       .092 

.090 

.088 

.086 

.084 

.082 

5,500 

.102       .100 

.098 

.096 

.094 

.092       .090 

.088 

.086 

.084 

.082 

.081 

6,000 

.100       .098 

.096 

.094 

.092 

.090       .088 

.086 

.084 

.082 

.081 

.080 

6,500 

.098       .096 

.094 

.092 

.090 

.  088       .  086 

.085 

.083 

.081 

.080 

.079 

7,000 

.  097       .  095 

.093 

.091 

.089* 

.087       .085       .084 

.082 

.080 

.079 

.078 

BAROMETERS   AND  MEASUREMENT   OF   ATMOSPHERIC   PRESSURE.      101 

TABLE  VI. — Determination  of  heights  by  the  barometer,  English  measures. 
[Abridged  from  the  Smithsonian  Tables.] 

29  90 
Values  of  60368  [1+0.0010195X361  log.  -^ 


Barometric 

pressure,  B.          •"" 

.02 

.03 

.04              .05 

.06 

.08 

.09 

Inchet. 

Feet.        Feet. 

Feet. 

Feet. 

Feet.         Feet. 

Feet. 

Feet.       Feet. 

Feet. 

17.00 

15,347 

15,331 

15,315 

15,299 

15,283        15,267 

16,261 

15,235     1.VJ19 

16,21 

.10 

15,  187 

15,  172 

15,  156 

15,  140 

15,124        15,108 

16,092 

15.081 

.20 

15,029 

15,013 

14.997 

14,982 

14,966        14,950 

11.  '<:;.» 

14,919     14,903 

14,887 

.30 

14,871 

14,866 

14,840 

14,824 

14,809        14,793 

14,777 

14,746 

.40 

14,715 

14,699 

14,684 

14,668 

14,652        14,637 

14,621 

14,606     14,590 

17.50 

14,559 

14,544 

14,528 

14,512 

14,  497        14.  481 

14,466 

14.4M      14,435 

14,420 

.60 

14.404 

14,389 

14,373 

14.3.58       14,342        14;327 

14,312 

14,296     14,281 

.70 

14,260 

14,235 

14,219 

14.204       14,189        14,173 

14,168 

14,143      14.12X' 

14.11 

.80 

14,097 

14,082 

14,067 

14,051       14,036       14,021 

14,008 

13.990     13,975 

13,960 

.90 

13,945 

13,930 

13,914 

13,899       13,884        13,869 

13,854 

13,839     13.824 

13,808 

18.00 

13,793 

13,778 

13.763 

13,748       13,733        13,718 

13,703 

13,688     13,673 

13,658 

.10 

13,643       13,628 

13,613 

13,598       13,583        13,568 

13,553     13,538     13,523 

18,888 

.20 

13.493        13,478 

13,463 

13,448  '     13,433        13,418 

13,404     13,389     13,374 

13.359 

.30 

13.344       13,329 

13,314 

13,300       13,285        13,270 

13,255     13,240     1 

13.211 

.40 

13,  196 

13,181 

13,  166 

13,152       13,137        13,122 

13,107 

13,093     13,078 

13,063 

18.50       13,049 

13,034 

13,019 

13,005       12,990        12,975 

12,961     12,946     12,931 

12.917 

.60       12,902       12,888 

12,  8V3 

12,858       12,844        12,829 

12.815     12,800     12,785 

12.771 

.70        12.7/ir.        12.742        12.727 

12,713       12,698        12,684 

12,669     12,655     12,640 

.80       12,611        12,597       12,583       12,568       12,554        12,539 

12,525     12,510     12,496 

12,483 

.90       12.407 

12,453 

12,438       12,424       12,410 

12,395 

12,381     12,367     12,352 

19.00       12.324 

12.310       12.295       12,281       12.267 

12.252 

12,238     12,224     12.210 

12,19.-) 

.10       12,181       12;  167       12,153       12,138       12,124        12,110 

12,096     12,082     12,068 

12,053 

.20       12,039       12,025 

12,011       11,997       11,983 

11,969 

11,954     11,940     11,926 

11,912 

.30 

11,898 

11,884 

11,870 

11,856 

11,842 

11,828 

11,814     11,800     11,786 

11.77. 

.40 

11,758 

11,744 

11,730 

11,716 

11,702 

11,688 

11,674     11,660     11,646 

11,632 

19.50 

11,618 

11,604 

11,590       11,576 

11,562        11,548 

11,534     11,520     11,507 

11,493 

.60       11.4V9       11.465       11.451       11.437 

11,423        11,410 

11,396     11,382     11,368 

11,354 

.70 

11,340       11,327       11,313        11,299 

11,285        11,272 

11,258     11,244     11,230 

11.217 

.80 

11,203       11.189 

11,175       11.162 

11,148        11,134 

11,121      11,107     11,093 

11,080 

.90 

11,06(3 

11,052 

11,039 

11,025 

11,011    1     10,998 

10,984     10,970     10,957 

10,943 

20.00 

10.930       10,916 

10,903 

10,889 

10,875        10,862 

10,848     10.83ff    10,821 

10,80* 

.10       10.794       10.781 

10.767       10,754 

10,740        10,727 

10,713     10,700     10,686 

lo.  ..7, 

.20 

10,659       10,646 

10,632       10,619 

10,605        10,592 

10,579     10,565     10,652 

«                    .30 
.40 

10,525 
10,391 

10,512 
10,378 

10,498       10,485 
10,365       10,352 

10,472        10,458 
10,338        10,325 

10,4*5X0.431     10,418 
10,312     10,298     10,285 

10.  405 
10.272 

20.50 

10,259       10,245 

10,232       10,219 

10,206        10,192 

10,179     10,166     10,153 

.60 

10,126       10,113 

10,100       10,087 

10,074 

10,060 

10,047     10,034 

10.021 

10.II-  - 

.70 

9,995        9,982 

9,988 

9,966 

9,942 

9,929 

9,916      9,903 

9,890 

.80 

9,864        9,851 

9,838 

9,825 

9,812 

9,799 

9,786         ,772 

9,759 

.90 

9,733        9,720 

9,707 

9,694 

9,681 

9,668 

9.655 

,642 

9,629 

9,617 

21.00 

9,604 

9.591 

9,578 

9,565 

9,552 

9,539 

9,526         .513 

9,500 

9,487 

.10 

9,  474        9,  462 

9,449        9,436 

9,423 

9,410 

9,397         ,384 

9,372 

.20 

9,346         9,333 

9,320        9.307 

9,295 

9,282 

9,269         ,256 

9.244 

9.231 

.30 

9.218         9.205 

9.193 

9,180 

9,167 

9,154 

9,142         ,129 

8,116 

9,103 

.40 

9.091 

9,078 

9,065 

9,053 

9,040 

9,027 

9,015 

9,002 

8,989 

8,977 

21.50 

8,964         8,951 

8,939 

8,926 

8,913 

8,901 

8,888       8,876 

8,881 

8,850 

.  tt). 

.70 

8,838        8,825 
8,  712         8.  700 

8,813 
8,687 

8,800 
8,675 

8,788 
8,662 

8,775 
8,650 

8,  762       8.  750 
8,637       8,625 

8,737 
8,8U 

8.725 
3,800 

.80 

.90 

8,587 
8,463 

8,575 
8,451 

8,562 
8,438 

8,550 
8,426 

8,538 
8,413 

8,525 
1  8,401 

8,513       8,500 
8,389      8,376 

8,488 

8,364 

22.00         8,339 
.10          8.216 

8,327 
8,204 

8,314 
8,191 

8,302 
8.179 

8,290 
8,167 

8,277 
8,154 

8,265      8,253 
8,142       8,130 

V     -'»" 

8,118 

8.228 

.20 

8,098 

8,061 

8,069         8:056 

8,044 

8,032 

8.020       8,008 

7.995 

7.'.^ 

.30 
.40 

7,971 
7,849 

7.959 
7.837 

7,947 
7,825 

7,935 
7,813 

7,922 
7,801 

7,910 
7,788 

7.818 

7,777 

7,880 
7,765 

7.874 
7,753 

22.50 
.60 

7,728 
7,608 

7,716 

7,692 
7,572 

7,680 
7,560 

7,668 
7,548 

7,656 

7,644 
7.524 

7,632 

7.51-; 

7,820 

7  BOO 

.70 
.80 
.90 

7,488 
7,368 
7,249 

7J  476 
7,288 

5.452 

7.214 

7.440 
7,321 

7,428 

7.  4i'. 

7,178 

7,288 

7,166 

TJtn 

7,155 

7J143 

102       BAROMETERS   AND  MEASUREMENT   OF   ATMOSPHERIC   PRESSURE. 
TABLE  VI. — Determination  of  heights  by  the  barometer,  English  measures— Con. 


Barometric 

.00 

.01 

.02 

.03 

.04 

.05 

.06 

.07 

r.os 

.09 

pressure,  B. 

Inches. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

23.00 

7,131 

7,119 

7,107 

7,096 

7,084 

7,072 

7,060 

7,048 

7,037 

7,025 

.10 

7,013 

7,001 

6,990 

6,978, 

-     6,966 

6,954 

6,943 

6,931 

6,919 

6,907 

.20 

6,896 

6,884 

6,872 

6,861 

6,849 

6,837 

6,825 

6,814 

6,802 

6,790 

.30 

6,779 

6,767 

6,755 

6,744 

6,732 

6,721 

6,709 

6,697 

6,686 

6,674 

.40 

6,662 

6,651 

6,639 

6,628 

6,616 

6,604 

6,593 

6,581 

6,  570 

6,558 

23.50 

6,546 

6,535 

6,523 

6,512 

6,500 

6,489 

6,477 

6,466 

6,454 

6,443 

.60 

6,431 

6,420 

6,408 

6,397 

6,385 

6,374 

6,362 

6,351 

6,339 

6,328 

.70 

6,316 

6,305 

6,293 

6,282 

6,270 

6,259 

6,247 

6,236 

6,225 

2,213 

.80 

6,202 

6,190 

6,179 

6,167 

6,156 

6.145 

6,133 

6,122 

6,110 

6,099 

.90 

6,088 

6,076 

6,065 

6,054 

6,042 

6,031 

6,020 

6,008 

5,997 

5,986 

24.00 

5,974 

5,963 

5,952 

5,940 

5,929 

5,918 

5,906 

5,895 

5,884 

5,872 

.10 

5,861 

5,850 

5,839 

5,827 

5,816 

5,805 

5,794 

5,782 

5.771 

5,760 

.20 

5,749 

5,737 

5,726 

5,715 

5,704 

5,693 

5,681 

5,670 

5,659 

5,648 

.30 

5,637 

5,625 

5,614 

5,603 

5,593 

5,581 

5,570 

5,558 

5,547 

5,536 

.40 

5,525 

5,514 

5,503 

5,492 

5,480 

5,469 

5,458 

5,447 

5,436 

5,425 

.  24.50 

5,414 

5,403 

5,392 

5,381 

5,369 

5,358 

5,347 

5,336 

5,325 

5,314 

.60 

5,303 

5,292 

5,281 

5,270 

5,259 

5,248 

5,237 

5,226 

5,215 

5,204 

.70 

5,193 

5,182 

5,171 

5,160 

5,149 

5,138 

5,127 

5,116 

5,105 

5,094 

.80 

5,083 

5,072 

5,061 

5,050 

5,039 

5,028 

5,017 

5,006 

,995 

4,985 

.90 

4,974 

4,963 

4,952 

4,941 

4,930 

4,919 

4,908 

4,897 

,886 

4,876 

25.00 

4,865 

4,854 

4,843 

4,832 

4,821 

4,810 

,800 

4,789 

,778 

4,767 

.10 

4,756 

4,745 

4,735 

4,724 

4,713 

4,702 

,691 

4,681 

,670 

4,659 

.20 

4,648 

4,637 

4,627 

4,616 

4,605 

4,594 

,584 

4,573 

,562 

4,551 

.30 

4,540 

4,530 

4,519 

4,508 

4,498 

4,487 

,476 

4,465 

,455 

4,444- 

.40 

4,433 

4,423 

4,412 

4,401 

4,391 

4,380 

,369 

4,358 

,348 

4,337 

25.50 

4,326 

4,316 

4,305 

4,295 

4,284 

4,273 

,263 

4,252 

4,241 

4,231 

.60 

4,220 

4,209 

4,199 

4,188 

4,178 

4,167 

,156 

4,146 

4,135 

4,125 

.70 

4,114 

4,104 

4.093 

4,082 

4,072 

4,061 

,051 

4,040 

4,030 

4,019 

.80 

4,009 

3,998 

3,988 

3,977 

3,966 

3,956 

3,945 

3,935 

3,924 

3,914 

.90 

3,903 

3,893 

3,882 

3,872 

3,861 

3,851 

3,841 

3,830 

3,820 

3,809 

26.00 

3,799 

3,788 

3,778 

3,767 

3,757 

3,746 

3,736 

3,726 

3,715 

3,705 

.10 

3,694 

3,684 

3,674 

3,663 

3,653 

3,642 

3,632 

3,622 

3,611 

3,601 

.20 

3,590 

3,580 

3,570 

3,559 

3,549 

3,539 

3,528 

3,518 

3,508 

3,497 

.30 

3,487 

3,477 

3,466 

3,456 

3,446 

3,435 

3,425 

3,415 

3,404 

3,394 

.40 

3,384 

3,373 

3,363 

3,353 

3,343 

3,332 

3,322 

3,312 

3,301 

3,291 

26.50 

3,281 

3,270 

3,260 

3,250 

3,240 

3,230 

3,219 

3,209 

3,199 

3,189 

.60 

3,179 

3,168 

3,158 

3,148 

3,138 

3,128 

3,117 

3,107 

3,097 

3,087 

.70 

3,077 

3,066 

3,056 

3,046 

3,036 

3,026 

3,016 

3,005 

2,995 

2,985 

.80 

2,975 

2,965 

2,955 

2,945 

2,934 

2,924 

2,914 

2,904 

2,894 

2,884 

.90 

2,874 

2,864 

2,854 

2,843 

2,833 

2,823 

2,813 

2,803 

2,793 

2,783 

27.00 

2,773 

2,763 

2,753 

2,743 

2,733 

2,723 

2,713 

2,703 

2,692 

2,682 

.10 

2,672 

2,662 

2,652 

2,642 

2,632 

2,622 

2,612 

2,602 

2,592 

2,582 

.20 

2,572 

2,562 

2,552 

2,542 

2,532 

2,522 

2,512 

2,502 

2,493 

2,483 

.30 

2,473 

2,463 

2,453 

2,443 

2,433 

2,423 

2,413 

2,403 

2,393 

2,383 

.40 

2,373 

2,363 

2,353 

2,343 

2,334 

2,324 

2,314 

2,  304 

2,294 

2,284 

27.50 

2,274 

2,264 

2,254 

2,245 

2,235 

2,225 

2,215 

2,205 

2,195 

2,185 

.60 

2,176 

2,166 

2,156 

2,146 

2,136 

2,126 

2,116 

2,107 

2,097 

2,087 

.70 

2,077 

2,067 

2,058 

2,048 

2,038 

2,028 

2,018 

2,009 

1,9£9 

1,989 

.80 

1,979 

1,970 

1,960 

1,950 

1,940 

1,930 

1,921 

1,911 

1,901 

1,891 

.90 

1,882 

1,872 

1,862 

1,852 

1,843 

1,833 

1,823 

1,814 

1,804 

1,794 

28.00 

1,784 

1,775 

1,765 

1,755 

1,746 

1,736 

1,726 

1,717 

1,707 

1,697 

.10 

1,688 

1,678 

1,668 

1,659 

1,649 

1,639 

1,630 

1,620 

1,610 

1,601 

.20 

1,591 

1,581 

1,572 

1,562 

1,552 

1,543 

1,533 

1,524 

1,514 

1,504 

.30 

1,495 

1,485 

1,476 

1,466 

1,456 

1,447 

1,437 

1,428 

1,418 

1,408 

.40 

1,399 

1,389 

1,380 

1,370 

1,361 

1,351 

1,342 

1,332 

1,322 

1,313 

28.50 

1,303 

1,294 

1,284 

1,275 

1,265 

1,256 

1,246 

1,237 

1,227 

1,218 

.60 

1,208 

1,199 

1,189 

1,180 

1,170 

1,161 

1,151 

,  1,142 

1,132 

1,123 

.70 

1,113 

1,104 

1,094 

1,085 

1,075 

1,066 

1,057^ 

1,047 

1,038 

1,028 

.80 

1,019 

1,009 

1,000 

990 

981 

972 

962 

953 

943 

934 

.90 

925 

915 

906 

896 

887 

878 

868 

859 

849 

840 

29.00 

831 

821 

812 

803 

794 

784 

775 

765 

756 

746 

.10 

737 

728 

718 

709 

700 

690 

681 

672 

663 

653 

.20 

644 

635 

625 

616 

607 

597 

588 

57ft 

.    570 

560 

.30 

551 

542 

532 

523 

514 

505 

495 

486 

477 

468 

.40 

458 

449 

440 

431 

421 

412 

403 

•     394 

384 

375 

BAROMETERS   AND  MEASUREMENT   OF   ATMOSPHERIC   PRESSURE.      103 
TABLE  VI. — Determination  of  heights  by  the  barometer,  English  measures — < 


Barometric 
pressure,  B. 

.00 

.01 

.02 

.03             .04 

.05           .06 

i 

.07 

.08 

Inches. 

Feet. 

Feet.  ' 

Feet. 

Feet.        Feet. 

Feet.        Feet. 

Feet. 

Feet.       Feet. 

29.50 

366 

357            348 

338            329 

320          311 

302 

292 

.00 

274 

256 

247            237 

228          219 

201            192 

.70 

182 

173             164 

155            146 

137          128 

118 

109            100 

.80 

+  91 

+  82         +73 

+  64         +55 

+  45       +  .36  :     +  27 

+  18        +9 

.90 

0 

-    9         -  18 

-  27         -36 

-  45       -  55 

-  64 

-  73 

30.00 

-  91 

-100        -109 

-118         -127 

-136       -145       -154 

-163         -172 

.10 

-181 

-190 

-199 

-208        -217 

-226       -235       -244 

-253         -262 

.30 

-271 

-280 

-289 

-298         -307 

-316       -325       -334 

-343         -352 

.30 

-361. 

-370 

-379 

-388         -397 

-406       -415       -424 

-433         -442 

.40 

-451 

-460 

-469 

-478         -486 

-495       -504 

-513 

-522         -531 

30.50 

-540 

-549 

-558 

-567         -576 

-585       -593 

-602 

-611         -620 

.60 

-629 

-638 

-647 

-656         -665 

-673       -682       -691 

-700        -709 

.70 

-718 

-727 

-735 

-744        -753 

-762       -771       -780 

-788        -797 

.80 

-806 

-815 

-824 

-833         -841 

-850       -859 

-868 

-877         -885 

104       BAROMETERS  AND  MEASUREMENT   OF   ATMOSPHERIC   PRESSURE. 

TABLE  VII. — Determination  of  heights  by  the  barometer,  English  measures. 
[Abridged  from  the  Smithsonian  Tables.] 
Term  for  temperature:  0.002039  (T-50°)  z. 

For  temperatures  {$£  »I  ?;}  the  values  are  *  be 


Mean  tempera- 
ture T. 

Approximate  elevations  obtained  from  Table  VI. 

1,000 

2,000  i  3,000 

4,000 

5,000 

6,000 

7,000 

8,000 

:    9,000 

10,000 

°F. 

°F. 

Feet. 

Feet.   Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

49 

51      2 

4      6 

8 

10 

12 

14     16 

18 

20 

48 

52      4 

8     12 

16 

20 

24 

29     33 

37 

41 

47 

53      6 

12     18 

24 

31 

37     43     49 

55 

61 

46 

54      8 

16     24 

33 

41 

49  '    57     05 

73 

82 

45 

55     10 

20     31     41 

51 

61     71     82 

92 

102 

44 

56     12 

24     37     49 

61 

73     86     98 

110 

122 

43 

57     14 

29     43 

57 

71 

86     100  ,    114 

128 

143 

42 

58     16 

33     49 

65 

82 

98  |    114     130 

147 

163 

41 

59     18 

37     55 

73 

92 

110     128  !    147 

165 

184 

40 

60 

^ 

20 

41     61 

82 

102 

122 

143 

163 

204- 

39 

61' 

22 

45     67 

90 

112 

135 

157 

179 

224 

38 

62     24 

49     73 

98 

122 

147     171     196 

220 

245 

37 

63     27 

53     80 

106 

133 

159     186     212 

239 

265 

36 

64 

29 

57     86 

114 

143 

171     200    228 

257 

285 

35 

65 

31 

61     92 

122 

153 

184     214 

245 

275 

306 

34 

66 

33 

65     98 

130    163 

196     228 

261 

294 

326 

33 

67     35 

69    104 

139    173 

208     243     277 

312 

347 

32 

68  i    37 

73 

110 

147     184 

220     257     294 

330 

367 

31 

69     39 

77 

116 

155     194 

232     271     310 

349 

387 

30 

70     41 

82 

122 

163     204 

245     285     326 

367 

408 

29 

71     43 

86 

128 

171     214 

257 

300    343 

385 

428 

28 

72     45 

90 

135 

179     224  1    269     314     359 

404 

449 

27 

73     47 

94 

141 

188     234  !    281     328     375 

422 

469 

26 

74     49 

98 

147 

1%     245     294  I    343     391 

440 

489 

25 

75 

51 

102 

153 

204     255 

306    357    408 

459 

510 

24 

76 

53 

106 

159 

212     265 

318 

371     424 

477 

530 

23 

77 

55 

110 

165 

220     275 

330 

385    440 

495 

551 

22 

78 

57 

114 

171 

228     285 

343 

400 

457 

514 

571 

21 

79 

59 

118 

177 

236     296 

355 

414 

473 

532 

591 

20 

80 

61 

122 

184 

245    306 

367 

428     489 

551 

612 

19 

81 

63 

126 

190 

253     316 

379     442 

506 

569 

632 

18 

82 

65 

130 

196 

261     326     391     457     522 

587 

652 

17     83 

67 

135 

202 

269    336     404    471    538 

606 

673 

16   -  84 

69 

139 

208 

277     347 

416     485 

555 

624 

693 

15     85 

71 

143 

214 

285    357 

428 

500 

571 

642 

714 

14     86 

73 

147 

220 

294     367 

440     514  ;    587 

661 

734 

13     87 

75 

151 

226 

302     377  j    453     528     604 

679 

754 

12     88 

77 

155 

232 

310     387  .,    46?     542     620 

697 

775 

11     89 

80 

159 

239 

318     398     477     557 

636 

716 

795 

10     90 

82 

163 

245 

326    408 

489  \   571    652 

734 

816 

9     91 

84 

167 

251 

334  !    418 

502    585 

669 

752 

836 

8     92 

86 

171 

257 

343     428     514     599 

685 

771 

856 

7     93 

88 

175 

263 

351     438     526     614 

701 

789 

877 

6     94 

90 

179 

269 

359     449 

538 

628 

718 

807 

897  • 

5     95 

92 

184 

275 

367     459  ; 

551 

642 

734 

826 

918 

4     96 

94 

188 

281 

375    469 

. 
563     657 

750 

844 

938 

3     97 

96 

192 

287 

383     479 

575     671 

767 

862 

958 

2     98 

98 

196 

294 

391     489     587     685     783 

881 

979 

1     99 

100 

200 

300 

400    500     599     699     799 

899 

999  ' 

0     100 

102 

204 

306 

408     510     612     714 

816 

918 

1,020 

INDEX. 

Paragraph. 

Actual  elevation  defined J  7 1 

Adjustment  of  barographs  at  high  elevations 154 

to  station  pressure 153, 172 

Attached  thermometer 8 

Atmospheric  pressure 4 

Authority  to  clean  barometers 187 

Air  and  moisture  in  barometer  tubes 130, 131 

can  not  be  perfectly  removed  from  barometer  tubes 133 

how  removed  from  barometer  tubes 133 

in  barometer  tubes . .  - 129 

vent  in  cistern  of  barometer Ill 

Tuch  cisterns 143, 144 

Aneroid  barographs,  Richard's 88 

barometer 6,  46-58 

"creeping  of" 56 

defects  in :  49, 55 

effects  of  temperature 48, 51 

Goldschmidt's 50 

how  adjusted 54 

,  in  determining  heights 105, 106 

reading,  how  made 52 

test  of  condition 58 

Barographs 68,69,191 

adjusted  for  high  elevations 154 

to  standard  pressure 153 

to  station  pressure 1  •"• 

aneroid,  Richard's 

clocks  for,  regulated  and  wound 157 

compensated  siphon,  Marvin  system 72-87 

check  observations 87 

cleaning  mercury 

dismantling 

magnifying  and  recording  mechanisms 

temperature  compensation ...  80, 81 

time  checks 

marker 85, 86 

corrections  for  errors  of 

exposure  of 89, 152, 176 

Foreman's 

forms  for,  changed 

refigured 

trimmed 

hourly  reading  of 

how  exposed 

105 


• 

106  INDEX. 

• 

Paragraph. 

Barographs,  Marvin's  normal. 71 

pens  for,  cleaned 159 

time  error,  checked 158, 192 

weighing 69 

Barometer,  air  vent  of  cistern Ill 

adjustment  of  cistern 97 

vernier 98 

aneroid 6 

at  elevated  stations,  how  handled 114 

boxes  for 91 

old  style 93 

marine 96 

care  and  preservation  of 108, 115 

change  or  substitution  of 116 

changes  in  location  of 148 

cisterns,  how  to  tighten  screws  of 1 136 

not  to  be  strained 16,  111,  114 

comparison  of,  private 194 

corrections,  for  capillarity 41 

imperfect  vacuum • 41 

instrumental  or  scale  error 41 

temperature 41 

correction  cards,  Form  No.  1059 — Met'l 42, 170 

diagonal 62 

dial.. 63 

errors  of 39,  40 

exposure  of 89, 152, 176 

Fortin,  Weather  Bureau  pattern 7 

frequent  cleaning  of  cisterns  to  be  avoided 117 

glass  cisterns  of,  washed 135 

tubes  of,  to  be  cleaned 128 

glycerin  „ ' 61 

Green 7 

holosteric 6 

how  carried Ill 

disposed  on  steamboats,  etc 147 

inverted 110,111 

set  and  observed 97 

to  open  cistern  of  leaky 126 

packed 145 

unpacked '    109 

Howson's 65 

instructions  for  cleaning 123-144, 187 

marine 26-31 

box  for 96 

handling  of 112 

magnifying 66,  67 

measurement  of  height  by 105, 107 

moving  and  packing  of 145 

normal 45 

not  to  be  changed  without  authority - 185 

parts  of  assembled 136 

pumping  of 89a 


9 

INDEX.  107 

Paragraph. 

Barometer,  readings  completely  corrected 171 

reduction  to  sea  level 103 

standard  gravity 102 

report  on  defective  or  unserviceable 150 

scale  of,  cleaned 128 

screwed  up  for  transportation 145 

screws  of  cistern  tightened  regularly 136, 141 

shipment  of  empty 151 

serviceable 145, 149 

siphon 3 

spiral : 62 

standard 45 

"station"  and  "extra"  defined 175 

Torricelli's 2 

tube,  air  removed  from 132 

tubes,  filling  of 32-37 

air-pump  method 36 

boiling  method 34 

funnel  method 33 

cleaning  of  large 35 

vacua  in 38 

Tuch,  cisterns  cleaned 143 

verticality  of 90, 92, 99 

water , 60 

when  to  make  change  in  location  of 186 

Boxes  for  barometers 91 

old  style 93 

Capacity  correction,  how  found 24 

Capillarity,  correction  for 174 

Cards,  barometer  correction,  Form  No.  1059— Met'l 42, 170 

Care  and  preservation  of  barometers 108 

Carrying  cases,  leather 146 

Change  in  location  of  barometer 148 

when  made 186 

or  substitution  of  barometers «* 116, 185, 186 

Cistern,  adjustment  of 97 

filtering  mercury  into 140 

fixed  and  adjustable •. 18 

glass  part  washed 135 

of  barometer 9 

not  to  be  strained Ill,  114 

frequently  cleaned 117 

screws  of,  tightened  regularly. .  .• • 136, 141 

'    straining  of 16,  111,  114 

Tuch,  instructions  for  cleaning 143 

pattern  described 

Cleaning  around  ivory  point , 

barometers,  instructions  for. 123-144, 187 

pens  for  barograph 

Clocks  for  barographs,  regulated  and  wound 

Comparative  barometer  readings 118-122, 18 

object  of H9 

recorded  on  Form  No.  1027— Met'l 166 


108  INDEX. 

Paragraph. 

Comparative  barometer  readings  when  levels  can  not  be  run 166 

readings,  interval  for,  after,  cleaning. 121, 142 

order  for  making,  on  removal  of  office 166 

Correction  of  barometer  readings,  example 171 

for  capacity 18,  24 

density  of  mercury 44 

error  of  barograph 160 

imperfect  vacuum 41, 174 

scale  errors,  capillarity,  etc 41, 174 

temperat.ure 41,  43, 17' 

tables  for 195 

provisional  removal 182;  198 

variable  removal 174, 182a 

Determination  of  heights  by  aneroids 104, 105, 107 

tables  for 199 

Diagonal  barometers • 62 

Dial  barometer 63 

Elevation  determined  by  competent  person 162, 165 

of  a  station  defined 161, 174 

barometer  above  sea  level,  determined  promptly 178 

actual 174 

station 174 

to  be  given  in  feet  and  decimals 162 

Empty  barometers,  shipment  of 151 

Engineer  to  be  informed  concerning  levels 165 

Error  of  barograph  clocks  checked 158 

barometers  as  shown  by  comparative  readings 116 

Exposure  of  barographs 89-152, 176 

barometers 89-152, 176 

Extra  barometers,  cisterns,  lowered  before  comparative  readings 177 

screwed  up 177 

Field  notes,  copy  of,  furnished 165 

"Fixed  point "  to  be  established 163 

Foreman's  barograph 70 

Forms  for  barographs  changed .*. 156 

trimmed 156 

refigured  for  barographs  at  elevated  stations 155 

Form  No.  1001— Met'l 171 

1027— Met'l 183 

1058— Met'l 165 

1059— Met'l 42, 170 

Fortin  barometers,  Weather  Bureau  pattern 7 

Glycerine  barometer 61 

Gravity,  influence  of,  on  barometric  observations,  tables 197 

reduction  to  standard 102, 197 

altitude  term 174 

latitude  term 174 

Green  barometer 7 

Heights  determined  by  aneroids 105, 107 

barometric  readings 104, 107 

tables  for,  determination  of,  by  barometer 199 

Holosteric  barometer 6 


INDEX.  109 

Paragraph. 

Howson's  barometer : 65 

Impure  mercury  not  to  be  used , 138 

Index  point  of  barometer 9 

Ink  for  barograph  pens 159 

Instructions  for  cleaning  barometers 123-144 

Interpolation  explained 196 

Interval  between  comparative  readings  on  removal  of  barometers 148, 166 

Inverting  barometers  at  elevated  station 114 

Ivory  point,  difficulty  of  cleaning  around 134 

of  barometer 10 

position  not  to  be  changed  in  cleaning  barometer 128, 136 

Leaks  in  cistern  of  barometer 141 

Leaky  barometer,  how  to  open  cistern  of 126 

Leather  carrying  cases 146 

Line  of  levels,  how  run 165 

need  not  be  run 167 

must  be  run 162 

Location  of  barometers  changed 148 

Magnifying  barometers,  comments  on 67 

Marine  barometer 26-31 

Marvin's  normal  barograph 71 

Mercury,  how  filtered 139 

how  purified 138 

impure,  disposed  of 189 

not  to  be  removed 190 

purity  of,  indicated 139 

tested 137 

requisition  for 188 

"Metallic  click"  in  barometer  tubes Ill,  113 

Moving  and  packing  barometer 109, 145 

Nomenclature 174 

Observations  of  pressure  comparable 173 

reduced  to  station  elevation 170 

regular  barometric 97,100,171,180 

Observed  reading  defined 174 

Packing  barometers 109, 145 

Pens  for  barograph  cleaned 159 

Plane  of  reference  to  be  adopted 164 

I'n-ssure  actual,  defined 174 

of  one  atmosphere 4 

reduced 174 

station 174 

tables  of,  corresponding  to  change  in  elevation  of  100  feet 198 

Private  barometers,  comparison  of 194 

Pumping  of  barometers 89a 

Purity  of  mercury,  how  indicated 139 

tested 137 

Reading  of  vernier - 12, 13, 14, 15 

Readings  of  barometer  corrected 

Recording  barometers  or  barographs ." 68,  69 

Reduction  to  sea  level  defined 

temperature  and  moisture  terms 103 


HO  INDEX. 

' 

Paragraph. 

Reduction  for  elevation  denned 174 

Removal  correction .  174, 182 

to  barometer,  provisional 182 

variable 182a 

Scale  of  barometer 11 

contracted 19 

polished 128 

errors  and  capillarity,  correction  for 174 

of  elevations  on  aneroid,  how  used 106 

feet  on  aneroid,  adjustable,  how  used 106 

only  approximate 105 

Sea  level,  reduction  of  barometer  to 103, 181 

Serviceable  barometers,  how  shipped 149 

Shipment  of  empty  barometers 151 

serviceable  barometers 149 

Siphon  barograph,  Marvin  system 72,  87 

barometer 3 

magnifying 66 

Spiral  barometers 62 

Standard  gravity,  reduction  to 102,  1.97 

Station  elevation 168, 169, 174 

Straining  cistern  of  barometer Ill,  114 

Sum  of  corrections  denned 174 

Surveyor  to  be  informed  concerning  levels 165 

Sympiesometer 64 

Temperature,  correction  for 41,  43, 174, 195 

Thermometer  attached 8 

Time  error  of  barographs  checked 158, 192 

Torricelli's  barometer 2 

"Total  correction "  denned 171, 174 

entered  on  Form  No.  1001— Met'l 171 

Tube  of  barometer  to  be  cleaned 128 

Unpacking  barometers Ill 

Unserviceable  barometers,  how  shipped '. 151 

Vernier  of  barometer -  12 

adjustment  of 98 

Verticality  of  barometers 90,  92,  99 

Water  barometer —        60 

Weighing  barographs 69 

Winding  clocks  for  barographs 146 

Wrench  required  for  cleaning  Tuch  cisterns 143 

. 

o 


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